frontends/va: Ignore requested size when creating VAEncCodedBufferType

frontends/va: Ignore requested size when creating VAEncCodedBufferType

The buffer data is not directly accessible to application and it's
internally used to only store VACodedBufferSegment struct.
Ignore the size requested by application and instead allocate
sizeof(VACodedBufferSegment). Use calloc to zero out the struct.

This can save significant amount of memory, for example FFmpeg
will request up to tens of MB for single buffer.

Cc: mesa-stable
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6462

Reviewed-by: Thong Thai <thong.thai@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24410>