drm/i915: Don't try to disable plane if it's already disabled

Check plane->fb in intel_disable_plane() to determine if the plane
is already disabled.

If the plane has an fb, then it must also have a crtc, so we can drop
the plane->crtc check and just call intel_enable_primary() directly.

v2: WARN and bail if the plane doesn't have a crtc when it should
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_sprite.c

@@ -863,7 +863,12 @@ intel_disable_plane(struct drm_plane *plane)
 	struct intel_plane *intel_plane = to_intel_plane(plane);
 	int ret = 0;
 
-	if (plane->crtc)
+	if (!plane->fb)
+		return 0;
+
+	if (WARN_ON(!plane->crtc))
+		return -EINVAL;
+
 	intel_enable_primary(plane->crtc);
 	intel_plane->disable_plane(plane, plane->crtc);