drm/i915/bxt: Broxton raises the maximum number of planes to 4

Pipe A and b have 4 planes.
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Reviewed-by: NAntti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -130,7 +130,7 @@ enum transcoder {
  *
  * This value doesn't count the cursor plane.
  */
-#define I915_MAX_PLANES	3
+#define I915_MAX_PLANES	4
 
 enum plane {
 	PLANE_A = 0,