drm/radeon/kms: make sure display hw is disabled when suspending

Disable the display hw when suspending.
Should fix bug:
https://bugzilla.redhat.com/show_bug.cgi?id=522393



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>