drm/i915: Check for id==PLANE_CURSOR instead of type==DRM_PLANE_TYPE_CURSOR

The VLV/CHV watermark calculation is really interested in the hardware
plane type rather than the plane type (which is more of a software
concept). Let's check plane->id rather plane->type.

No functional changes.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303151928.23053-3-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_pm.c

@@ -1039,7 +1039,7 @@ static uint16_t vlv_compute_wm_level(const struct intel_crtc_state *crtc_state,
 	if (WARN_ON(htotal == 0))
 		htotal = 1;
 
-	if (plane->base.type == DRM_PLANE_TYPE_CURSOR) {
+	if (plane->id == PLANE_CURSOR) {
 		/*
 		 * FIXME the formula gives values that are
 		 * too big for the cursor FIFO, and hence we