x86/hpeldsp: fix loop in {avg,avg_no_rnd}_pixels16_x2_mmx

Handle it inside the __asm__() block.
Fixes fate-vc1_ilaced_twomv when using the gcc-usan toolchain.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>
