drm: Add dev->vblank_disable_immediate flag

Add a flag to drm_device which will cause the vblank code to bypass the
disable timer and always disable the vblank interrupt immediately when
the last reference is dropped.

v2: Add some notes about the flag to the kernel doc

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 99145c4..8dbcc3f 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -994,7 +994,7 @@
 
 	/* Last user schedules interrupt disable */
 	if (atomic_dec_and_test(&vblank->refcount)) {
-		if (drm_vblank_offdelay < 0)
+		if (dev->vblank_disable_immediate || drm_vblank_offdelay < 0)
 			vblank_disable_fn((unsigned long)vblank);
 		else if (drm_vblank_offdelay > 0)
 			mod_timer(&vblank->disable_timer,