drm: Clarify the role of plane_state argument to drm_simple update().

Like the atomic update hook it's wrapping, the plane_state is the old
one, and the new one is in plane->state.  Both msxfb and tinydrm use
it correctly, but I mistook it for the new state in pl111 due to its
naming.
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170320233615.5242-3-eric@anholt.netReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_simple_kms_helper.c

@@ -114,7 +114,7 @@ static int drm_simple_kms_plane_atomic_check(struct drm_plane *plane,
 }
 
 static void drm_simple_kms_plane_atomic_update(struct drm_plane *plane,
-					struct drm_plane_state *pstate)
+					struct drm_plane_state *old_pstate)
 {
 	struct drm_simple_display_pipe *pipe;
 
@@ -122,7 +122,7 @@ static void drm_simple_kms_plane_atomic_update(struct drm_plane *plane,
 	if (!pipe->funcs || !pipe->funcs->update)
 		return;
 
-	pipe->funcs->update(pipe, pstate);
+	pipe->funcs->update(pipe, old_pstate);
 }
 
 static int drm_simple_kms_plane_prepare_fb(struct drm_plane *plane,

include/drm/drm_simple_kms_helper.h

@@ -72,7 +72,7 @@ struct drm_simple_display_pipe_funcs {
 	 * the hardware lacks vblank support entirely.
 	 */
 	void (*update)(struct drm_simple_display_pipe *pipe,
-		       struct drm_plane_state *plane_state);
+		       struct drm_plane_state *old_plane_state);
 
 	/**
 	 * @prepare_fb: