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 777e3cbc
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>

drivers/gpu/drm/radeon/atombios_crtc.c

@@ -275,6 +275,8 @@ void atombios_crtc_dpms(struct drm_crtc *crtc, int mode)
 			atombios_enable_crtc_memreq(crtc, ATOM_ENABLE);
 		atombios_blank_crtc(crtc, ATOM_DISABLE);
 		drm_vblank_post_modeset(dev, radeon_crtc->crtc_id);
+		/* Make sure vblank interrupt is still enabled if needed */
+		radeon_irq_set(rdev);
 		radeon_crtc_load_lut(crtc);
 		break;
 	case DRM_MODE_DPMS_STANDBY:

drivers/gpu/drm/radeon/radeon_legacy_crtc.c

@@ -331,6 +331,8 @@ static void radeon_crtc_dpms(struct drm_crtc *crtc, int mode)
 			WREG32_P(RADEON_CRTC_EXT_CNTL, crtc_ext_cntl, ~(mask | crtc_ext_cntl));
 		}
 		drm_vblank_post_modeset(dev, radeon_crtc->crtc_id);
+		/* Make sure vblank interrupt is still enabled if needed */
+		radeon_irq_set(rdev);
 		radeon_crtc_load_lut(crtc);
 		break;
 	case DRM_MODE_DPMS_STANDBY: