radeonsi/vcn: change max_poc to fixed value for hevc encoder.

radeonsi/vcn: change max_poc to fixed value for hevc encoder.

problem: max_poc means the number of bits used in poc lsb
         in slice header, and it should not be related to GOP
	 size. When large GOP size used, it could generate
	 corrupted video, as the POC could not be correctly
	 decoded.

solution: use fixed value of max_poc (16) for now.

Cc: mesa-stable
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25214>