radeonsi/vcn: set bitstream buffer size to encoded bitstream size

radeonsi/vcn: set bitstream buffer size to encoded bitstream size

initial bitstream size was set to width * height * 2 which is
larger than yuv size. set initial bitstream size to encoded
bitstream size approximately to optimize memory consumption.
This is just an initial size setting, it will get resized later
if it's not big enough. As a result of this change, we don't need to
allocate super big size at the every beginning. Only allocate
big size when needed in order to save some memory

Signed-off-by: Sajeesh Sidharthan <sajeesh.sidharthan@amd.com>
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Acked-by: Veerabadhran Gopalakrishnan <Veerabadhran.Gopalakrishnan@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21918>