drm/radeon/kms/pm: fix segfault in clock code

Make sure we have a crtc assigned to the encoder
before dereferencing it.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>

drivers/gpu/drm/radeon/radeon_pm.c

@@ -279,6 +279,7 @@ void radeon_pm_compute_clocks(struct radeon_device *rdev)
 	list_for_each_entry(connector,
 		&ddev->mode_config.connector_list, head) {
 		if (connector->encoder &&
+		    connector->encoder->crtc &&
 		    connector->dpms != DRM_MODE_DPMS_OFF) {
 			radeon_crtc = to_radeon_crtc(connector->encoder->crtc);
 			rdev->pm.active_crtcs |= (1 << radeon_crtc->crtc_id);