radeon/vcn: enable rate control for hevc encoding

Set cu_qp_delta_enable_flag on when rate control is enabled, and set it
off when rate control is disabled (e.g. constant qp).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110673


Cc: mesa-stable@lists.freedesktop.org

V2: fix typo and add bugzilla info

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
(cherry picked from commit b0626c1f)