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