vp9/x86: rename ff_avg[48]_sse to ff_avg[48]_mmxext

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>