vmx: implement fast path vmx_composite_src_x888_8888

vmx: implement fast path vmx_composite_src_x888_8888

It was benchmarked against commid id 2be523b from pixman/master

POWER8, 8 cores, 3.4GHz, RHEL 7.1 ppc64le.
reference memcpy speed = 24764.8MB/s (6191.2MP/s for 32bpp fills)

                Before           After           Change
              ---------------------------------------------
L1              1115.4          5006.49         +348.85%
L2              1112.26         4338.01         +290.02%
M               1110.54         2524.15         +127.29%
HT              745.41          1140.03         +52.94%
VT              749.03          1287.13         +71.84%
R               423.91          547.6           +29.18%
RT              205.79          194.98          -5.25%
Kops/s          1414            1361            -3.75%

cairo trimmed benchmarks :

Speedups
========
t-gnome-system-monitor  1402.62  -> 1212.75 :  1.16x
   t-firefox-asteroids   533.92  ->  474.50 :  1.13x

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>