drm/xe/mmio: move mmio_fini over to devm

Not valid to touch mmio once the device is removed, so make sure we
unmap on removal and not just when driver instance goes away. Also set
the mmio pointers to NULL to hopefully catch such issues more easily.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240522102143.128069-32-matthew.auld@intel.com