drm/radeon: fix MC blackout on evergreen+

Force the crtc mem requests on/off immediately rather
than waiting for the double buffered updates to kick in.
Seems we miss the update in certain conditions.  Also
handle the DCE6 case.

Signed-off-by: Christopher Staite <chris@yourdreamnet.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org