drm: fix a typo in a comment

Spotted while reviewing the DRM changes in Linux 3.18 for LinuxFR.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NMartin Peres <martin.peres@free.fr>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_irq.c

@@ -166,7 +166,7 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
 	spin_lock_irqsave(&dev->vblank_time_lock, irqflags);
 
 	/*
-	 * If the vblank interrupt was already disbled update the count
+	 * If the vblank interrupt was already disabled update the count
 	 * and timestamp to maintain the appearance that the counter
 	 * has been ticking all along until this time. This makes the
 	 * count account for the entire time between drm_vblank_on() and