drm/i915: Fix gen3/4 vblank counter wraparound

When the hardware frame counter reads 0xffffff and we're already past
vblank start, we'd return 0x1000000 as the vblank counter value. Once
we'd cross into the next frame's active portion, the vblank counter
would wrap to 0. So we're reporting two different vblank counter values
for the same frame.

Fix the problem by masking the cooked value by 0xffffff to make sure
the counter wraps already after vblank start.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -583,7 +583,7 @@ static u32 i915_get_vblank_counter(struct drm_device *dev, int pipe)
 	 * Cook up a vblank counter by also checking the pixel
 	 * counter against vblank start.
 	 */
-	return ((high1 << 8) | low) + (pixel >= vbl_start);
+	return (((high1 << 8) | low) + (pixel >= vbl_start)) & 0xffffff;
 }
 
 static u32 gm45_get_vblank_counter(struct drm_device *dev, int pipe)