radeon/vcn2: enable rate control for hevc encoding

Based on b0626c1f ("radeon/vcn: enable rate control for hevc encoding").

Reviewed-by: Boyuan Zhang <boyuan.zhang@amd.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2225
Fixes: 587b9c5d ("radeon/vcn: implement vcn 2.0 encode")
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3134>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3134>
(cherry picked from commit 9c2a3b4e)