drm/i915/bdw: Add comment about gen8 HWS PGA

This confused me some many times that I think it is appropriate to add a
small comment to instruct the reader of the code that it is indeed doing
what it is supposed to do.
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -965,6 +965,7 @@ void intel_ring_setup_status_page(struct intel_ring_buffer *ring)
 	} else if (IS_GEN6(ring->dev)) {
 		mmio = RING_HWS_PGA_GEN6(ring->mmio_base);
 	} else {
+		/* XXX: gen8 returns to sanity */
 		mmio = RING_HWS_PGA(ring->mmio_base);
 	}