gallium: Parse packed HEVC SPS encode header for crop parameters

gallium: Parse packed HEVC SPS encode header for crop parameters

The crop / conformance window parameters are set by ffmpeg but they only
seem to be made available in packed headers. This commit copies the H265
header parsing code from st/omx (planning in the future to move this
code to a common place to be shared by the different state trackers) in
order to grab the crop parameters

Signed-off-by: Thong Thai <thong.thai@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4184>