drm: Update obsolete information from {enable/disable}_vblank hooks.

Since commit 4dfd64862f ("drm: Use vblank timestamps to guesstimate
how many vblanks were missed"), the DRM framework can cope with devices
that don't have a hardware counter for vsync events without having
to keep the vsync interrupts enabled all the time. Drivers handling
such hardware should use drm_vblank_no_hw_counter() function for
their ->get_vblank_counter hook.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464795342-32297-1-git-send-email-Liviu.Dudau@arm.com

include/drm/drmP.h

@@ -430,7 +430,7 @@ struct drm_driver {
 	 *
 	 * Driver callback for fetching a raw hardware vblank counter for @crtc.
 	 * If a device doesn't have a hardware counter, the driver can simply
-	 * return the value of drm_vblank_count. The DRM core will account for
+	 * use drm_vblank_no_hw_counter() function. The DRM core will account for
 	 * missed vblank events while interrupts where disabled based on system
 	 * timestamps.
 	 *
@@ -448,8 +448,8 @@ struct drm_driver {
 	 * @pipe: which irq to enable
 	 *
 	 * Enable vblank interrupts for @crtc.  If the device doesn't have
-	 * a hardware vblank counter, this routine should be a no-op, since
-	 * interrupts will have to stay on to keep the count accurate.
+	 * a hardware vblank counter, the driver should use the
+	 * drm_vblank_no_hw_counter() function that keeps a virtual counter.
 	 *
 	 * RETURNS
 	 * Zero on success, appropriate errno if the given @crtc's vblank
@@ -463,8 +463,8 @@ struct drm_driver {
 	 * @pipe: which irq to enable
 	 *
 	 * Disable vblank interrupts for @crtc.  If the device doesn't have
-	 * a hardware vblank counter, this routine should be a no-op, since
-	 * interrupts will have to stay on to keep the count accurate.
+	 * a hardware vblank counter, the driver should use the
+	 * drm_vblank_no_hw_counter() function that keeps a virtual counter.
 	 */
 	void (*disable_vblank) (struct drm_device *dev, unsigned int pipe);