drm/radeon/radeon_connectors.c: add a NULL test before dereference

The encoder variable can be NULL in this function so I believe it should
be checked before dereference.

Coverity CID: 13253

[airlied: extremely unlikely to happen]
Signed-off-by: Darren Jenkins <darrenrjenkins@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -615,7 +615,7 @@ static enum drm_connector_status radeon_vga_detect(struct drm_connector *connect
 				ret = connector_status_connected;
 		}
 	} else {
-		if (radeon_connector->dac_load_detect) {
+		if (radeon_connector->dac_load_detect && encoder) {
 			encoder_funcs = encoder->helper_private;
 			ret = encoder_funcs->detect(encoder, connector);
 		}