drm/radeon: disable the crtcs in mc_stop (evergreen+) (v2)

Just disabling the mem requests should be enough, but
that doesn't seem to work correctly on efi systems.

May fix:
https://bugs.freedesktop.org/show_bug.cgi?id=57567
https://bugs.freedesktop.org/show_bug.cgi?id=43655
https://bugzilla.kernel.org/show_bug.cgi?id=56441

v2: blank displays first, then disable.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org