pavgb is an sse integer instruction, so the mmxext flag is enough
Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Cycle counts for large MCs (old -> new on ped1080p.webm, mx!=0&&my!=0):
16x8:    876 ->   870  (0.7%)
16x16:  1444 ->  1435  (0.7%)
16x32:  2784 ->  2748  (1.3%)
32x16:  2455 ->  2349  (4.5%)
32x32:  4641 ->  4084 (13.6%)
32x64:  9200 ->  7834 (17.4%)
64x32:  8980 ->  7197 (24.8%)
64x64: 17330 -> 13796 (25.6%)
Total decoding time goes from 9.326sec to 9.182sec.

(And in future, loopfilter or intra pred could be put in their own
respective files also.)