drm/xe/ggtt: use drm_dev_enter to mark device section

Device can be hotunplugged before we start destroying gem objects. In
such a case don't touch the GGTT entries, trigger any invalidations or
mess around with rpm.  This should already be taken care of when
removing the device, we just need to take care of dealing with the
software state, like removing the mm node.

v2: (Andrzej)
  - Avoid some duplication by tracking the bound status and checking
    that instead.

References: drm/xe/kernel#1717


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>
Reviewed-by: Jagmeet Randhawa <jagmeet.randhawa@intel.com>