dsputil: add missing HAVE_YASM guard

Fix compile error under
"--disable-optimizations --disable-yasm --disable-inline-asm"

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
dsputil: x86: Correct the number of registers used in put_no_rnd_pixels16_l2

put_no_rnd_pixels16_l2 allocated 5 instead of 6 registers.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>