util/rbsp: Fill bits twice if reading more than 16 bits

util/rbsp: Fill bits twice if reading more than 16 bits

vl_rbsp_fillbits may fill less than 32 bits if it removes emulation
prevention bytes, but will fill at least 16 bits. We need to call it
twice when reading more than 16 bits.

This fixes parsing H264 SPS packed header in va frontend when emulation
prevention bytes are at position where 32 bit values are read.

Cc: mesa-stable
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26276>
(cherry picked from commit 73d69ef1e6b013f923ec459c789976570baaf447)