drm/amdgpu: enable mcbp by default on gfx9

It's required for high priority queues.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2535


Reviewed-and-tested-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>