drm/radeon: Ensure vblank interrupt is enabled on DPMS transition to on

drm/radeon: Ensure vblank interrupt is enabled on DPMS transition to on

NOTE: This patch only applies to 4.5.y or older kernels. With newer
kernels, this problem cannot happen because the driver now uses
drm_crtc_vblank_on/off instead of drm_vblank_pre/post_modeset[0]. I
consider this patch safer for older kernels than backporting the API
change, because drm_crtc_vblank_on/off had various issues in older
kernels, and I'm not sure all fixes for those have been backported to
all stable branches where this patch could be applied.

    ---------------------

Fixes the vblank interrupt being disabled when it should be on, which
can cause at least the following symptoms:

* Hangs when running 'xset dpms force off' in a GNOME session with
  gnome-shell using DRI2.
* RandR 1.4 slave outputs freezing with garbage displayed using
  xf86-video-ati 7.8.0 or newer.

[0] See upstream commit:

commit 777e3cbc791f131806d9bf24b3325637c7fc228d
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jan 21 11:08:57 2016 +0100

    drm/radeon: Switch to drm_vblank_on/off

Reported-and-Tested-by: Max Staudt <mstaudt@suse.de>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>