MIPS: DSPr2: Added fast-paths for SRC operation.

MIPS: DSPr2: Added fast-paths for SRC operation.

Following fast-path functions are implemented (routines 4, 5 and 6 utilize
same fast-memcpy routine):
    1. src_x888_8888
    2. src_8888_0565
    3. src_0565_8888
    4. src_0565_0565
    5. src_8888_8888
    6. src_0888_0888

Performance numbers before/after on MIPS-74kc @ 1GHz

Referent (before):

lowlevel-blt-bench:
        src_x888_8888 =  L1: 199.35  L2:  96.54  M: 18.87 (100.68%)  HT: 17.12  VT: 16.24  R: 15.43  RT:  9.33 (  61Kops/s)
        src_8888_0565 =  L1:  71.22  L2:  51.95  M: 24.19 ( 96.17%)  HT: 20.71  VT: 19.92  R: 18.15  RT:  9.92 (  63Kops/s)
        src_0565_8888 =  L1:  38.82  L2:  36.22  M: 18.60 ( 73.95%)  HT: 14.47  VT: 13.19  R: 12.97  RT:  6.61 (  49Kops/s)
        src_0565_0565 =  L1: 286.05  L2: 155.02  M: 37.68 (100.54%)  HT: 31.08  VT: 28.07  R: 26.26  RT: 11.93 (  68Kops/s)
        src_8888_8888 =  L1: 454.32  L2: 139.15  M: 19.30 (102.98%)  HT: 17.73  VT: 16.08  R: 16.62  RT: 10.45 (  64Kops/s)
        src_0888_0888 =  L1: 190.47  L2: 106.14  M: 25.26 (101.08%)  HT: 21.88  VT: 20.32  R: 18.83  RT: 10.10 (  63Kops/s)
cairo-perf-trace:
[ # ]  backend                         test   min(s) median(s) stddev. count
[ # ]    image: pixman 0.25.1
[  0]    image            firefox-asteroids  421.215  421.325   0.01%    4/6
[  1]    image         firefox-planet-gnome  647.708  648.486   0.13%    6/6
[  2]    image         gnome-system-monitor  276.073  277.506   0.38%    6/6
[  3]    image           gnome-terminal-vim  263.866  265.229   0.39%    6/6
[  4]    image                      poppler  123.576  124.003   0.15%    6/6

Optimized (with these optimizations):

lowlevel-blt-bench:
        src_x888_8888 =  L1: 369.50  L2:  99.37  M: 27.19 (145.07%)  HT: 20.24  VT: 19.48  R: 19.00  RT: 10.22 (  63Kops/s)
        src_8888_0565 =  L1: 105.65  L2:  67.87  M: 25.41 (101.00%)  HT: 20.78  VT: 19.84  R: 18.52  RT:  9.81 (  63Kops/s)
        src_0565_8888 =  L1:  77.10  L2:  63.04  M: 23.37 ( 92.90%)  HT: 20.29  VT: 19.37  R: 18.14  RT: 10.02 (  63Kops/s)
        src_0565_0565 =  L1: 519.02  L2: 241.32  M: 62.35 (166.34%)  HT: 33.74  VT: 27.63  R: 26.12  RT: 11.70 (  67Kops/s)
        src_8888_8888 =  L1: 390.48  L2: 113.99  M: 30.32 (161.77%)  HT: 19.55  VT: 17.05  R: 17.13  RT: 10.19 (  63Kops/s)
        src_0888_0888 =  L1: 349.74  L2: 156.68  M: 40.68 (162.78%)  HT: 25.58  VT: 20.57  R: 20.20  RT:  9.96 (  63Kops/s)
cairo-perf-trace:
[ # ]  backend                         test   min(s) median(s) stddev. count
[ # ]    image: pixman 0.25.1
[  0]    image            firefox-asteroids  400.050  400.308   0.04%    6/6
[  1]    image         firefox-planet-gnome  628.978  629.364   0.07%    6/6
[  2]    image         gnome-system-monitor  270.247  270.313   0.03%    6/6
[  3]    image           gnome-terminal-vim  256.413  257.641   0.21%    6/6
[  4]    image                      poppler  119.540  120.023   0.21%    6/6