commit 62444b74 upstream.

- Stop the displays from accessing the FB
- Block CPU access
- Turn off MC client access

This should fix issues some users have seen, especially
with UEFI, when changing the MC FB location that result
in hangs or display corruption.

v2: fix crtc enabled check noticed by Luca Tettamanti
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2:
 - Drop DCE6 cases
 - Call evergreen_mc_wait_for_idle() directly
 - Add dce4_wait_for_vblank() (commits 3ae19b75
   and 4a15903d) and call it directly
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>