OMAPDSS: fix potential NULL pointer ref in OCP_ERR handling path

The dispc's error handler tries to disable all outputs when OCP_ERR
happens. However, the code doesn't check if there actually is a display
on each particular output, nor if there's a driver for the display. This
may lead to NULL pointer reference.
Signed-off-by: Rob Clark <rob@ti.com>
[tomi.valkeinen@ti.com: added patch description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/dss/dispc.c

@@ -3201,7 +3201,8 @@ static void dispc_error_worker(struct work_struct *work)
 		for (i = 0; i < omap_dss_get_num_overlay_managers(); ++i) {
 			struct omap_overlay_manager *mgr;
 			mgr = omap_dss_get_overlay_manager(i);
-			mgr->device->driver->disable(mgr->device);
+			if (mgr->device && mgr->device->driver)
+				mgr->device->driver->disable(mgr->device);
 		}
 	}