drm/radeon: disable the crtcs in mc_stop (r5xx-r7xx) (v2)

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

v2: blank displays first, then disable.

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