drm/omap: cleanup omap_plane_atomic_check()

Clean up omap_plane_atomic_check() with:

- Check state->fb first. If no fb, return 0.
- use drm_atomic_get_existing_crtc_state() instead of
  drm_atomic_get_crtc_state()
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/omap_plane.c

@@ -157,12 +157,17 @@ static int omap_plane_atomic_check(struct drm_plane *plane,
 {
 	struct drm_crtc_state *crtc_state;
 
-	if (!state->crtc)
+	if (!state->fb)
 		return 0;
 
-	crtc_state = drm_atomic_get_crtc_state(state->state, state->crtc);
-	if (IS_ERR(crtc_state))
-		return PTR_ERR(crtc_state);
+	/* crtc should only be NULL when disabling (i.e., !state->fb) */
+	if (WARN_ON(!state->crtc))
+		return 0;
+
+	crtc_state = drm_atomic_get_existing_crtc_state(state->state, state->crtc);
+	/* we should have a crtc state if the plane is attached to a crtc */
+	if (WARN_ON(!crtc_state))
+		return 0;
 
 	if (!crtc_state->enable)
 		return 0;
@@ -176,11 +181,9 @@ static int omap_plane_atomic_check(struct drm_plane *plane,
 	if (state->crtc_y + state->crtc_h > crtc_state->adjusted_mode.vdisplay)
 		return -EINVAL;
 
-	if (state->fb) {
 	if (state->rotation != DRM_ROTATE_0 &&
 	    !omap_framebuffer_supports_rotation(state->fb))
 		return -EINVAL;
-	}
 
 	return 0;
 }