drm/radeon: don't probe MST on hw we don't support it on

If you do radeon.mst=1 on a gpu without mst hw, and then
plug some mst hw it will oops instead of falling back.

So check we have DCE5 at least before proceeding.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Christian König <christian.koenig@amd.com>