For delayed BO release i915_ttm_delete_mem_notify()
gets called twice, once with proper bo->resource and
another time with NULL. We shouldn't do anything for
the 2nd time as we already cleaned up the obj once.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/6850
Fixes: ad74457a ("drm/i915/dgfx: Release mmap on rpm suspend")
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220920170628.3391-1-nirmoy.das@intel.com
(cherry picked from commit fb781898)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>