drm: zynqmp_kms: Unplug DRM device before removal

Prevent userspace accesses to the DRM device from causing
use-after-frees by unplugging the device before we remove it. This
causes any further userspace accesses to result in an error without
further calls into this driver's internals.

Fixes: d76271d ("drm: xlnx: DRM/KMS driver for Xilinx ZynqMP DisplayPort Subsystem")
Closes: https://lore.kernel.org/dri-devel/4d8f4c9b-2efb-4774-9a37-2f257f79b2c9@linux.dev/
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809193600.3360015-2-sean.anderson@linux.dev

drivers/gpu/drm/xlnx/zynqmp_kms.c

@@ -538,7 +538,7 @@ void zynqmp_dpsub_drm_cleanup(struct zynqmp_dpsub *dpsub)
 {
 	struct drm_device *drm = &dpsub->drm->dev;
 
-	drm_dev_unregister(drm);
+	drm_dev_unplug(drm);
 	drm_atomic_helper_shutdown(drm);
 	drm_encoder_cleanup(&dpsub->drm->encoder);
 	drm_kms_helper_poll_fini(drm);