drm/panthor: Reset the FW VM to NULL on unplug

This way get NULL derefs instead of use-after-free if the FW VM is
referenced after the device has been unplugged.

Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Steven Price <[email protected]>
Acked-by: Liviu Dudau <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: bbrezillon

drivers/gpu/drm/panthor/panthor_fw.c

@@ -1142,6 +1142,7 @@ void panthor_fw_unplug(struct panthor_device *ptdev)
 	 * state to keep the active_refcnt balanced.
 	 */
 	panthor_vm_put(ptdev->fw->vm);
+	ptdev->fw->vm = NULL;
 
 	panthor_gpu_power_off(ptdev, L2, ptdev->gpu_info.l2_present, 20000);
 }