drm/i915/gem: Retire directly for mmap-offset shrinking

Now that we can retire without taking struct_mutex, we can do so to
handle shrinking the mmap-offset space after an allocation failure.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004134015.13204-11-chris@chris-wilson.co.uk