drm/i915: The shrinker already acquires struct_mutex, so call it unlocked

drm/i915: The shrinker already acquires struct_mutex, so call it unlocked

The shrinker is prepared to be called unlocked (and at other times with
struct_mutex held for DIRECT_RECLAIM) so we can skip acquiring the
struct_mutex prior to calling the shrinker during freeze. This improves
our ability to shrink as we can be more aggressive when we know the
caller isn't holding struct_mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407102552.5781-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>