drm/radeon: properly handle crtc powergating

commit 6c0ae2ab85fc4a95cae82047a7db1f688a7737ab upstream.

Need to make sure the crtc is gated on before modesetting.
Explicitly gate the crtc on in prepare() and set a flag
so that the dpms functions don't gate it off during
mode set.

Noticed by sylware on IRC.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>