vmx: implement fast path vmx_composite_over_n_8_8888

vmx: implement fast path vmx_composite_over_n_8_8888

POWER8, 8 cores, 3.4GHz, RHEL 7.2 ppc64le.

reference memcpy speed = 25008.9MB/s (6252.2MP/s for 32bpp fills)

                Before         After           Change
              ---------------------------------------------
L1              91.32          182.84         +100.22%
L2              94.94          182.83         +92.57%
M               95.55          181.51         +89.96%
HT              88.96          162.09         +82.21%
VT              87.4           168.35         +92.62%
R               83.37          146.23         +75.40%
RT              66.4           91.5           +37.80%
Kops/s          683            859            +25.77%

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>