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 <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502183813.1612017-4-boris.brezillon@collabora.com

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);
 }