st/mesa: fix 2 crashes in st_tgsi_lower_yuv

src/mesa/state_tracker/st_tgsi_lower_yuv.c:68: void reg_dst(struct
 tgsi_full_dst_register *, const struct tgsi_full_dst_register *, unsigned
 int): assertion "dst->Register.WriteMask" failed

The second crash was due to insufficient allocated size for TGSI
instructions.

Cc: 19.0 19.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
(cherry picked from commit 83435e74)