drm/plane-helper: Skip prepare_fb/cleanup_fb when newfb==oldfb

When commiting a plane update where the framebuffer doesn't change, we
can skip the prepare_fb/cleanup_fb steps.  This also allows us to avoid
an unnecessary vblank wait at the end of the operation when we're just
moving a plane and not changing its image (e.g., for a cursor).

At the moment, i915 is the only upstream driver using the transitional
plane helpers, and thus the only driver affected by this change.

Note that this replicates a corresponding change in the atomic helpers
implemented in

commit ab58e3384b9f9863bfd029b458ff337d381bf6d2
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Nov 24 20:42:42 2014 +0100

    drm/atomic-helper: Skip vblank waits for unchanged fbs

Reported-by: Jeremiah Mahler <jmmahler@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88540
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by: Tested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_plane_helper.c

@@ -435,7 +435,8 @@ int drm_plane_helper_commit(struct drm_plane *plane,
 			goto out;
 	}
 
-	if (plane_funcs->prepare_fb && plane_state->fb) {
+	if (plane_funcs->prepare_fb && plane_state->fb &&
+	    plane_state->fb != old_fb) {
 		ret = plane_funcs->prepare_fb(plane, plane_state->fb);
 		if (ret)
 			goto out;
@@ -456,6 +457,13 @@ int drm_plane_helper_commit(struct drm_plane *plane,
 			crtc_funcs[i]->atomic_flush(crtc[i]);
 	}
 
+	/*
+	 * If we only moved the plane and didn't change fb's, there's no need to
+	 * wait for vblank.
+	 */
+	if (plane->state->fb == old_fb)
+		goto out;
+
 	for (i = 0; i < 2; i++) {
 		if (!crtc[i])
 			continue;