drm/i915: Stop needlessly acquiring wakeref for debugfs/drop_caches_set

We only need to acquire a wakeref for ourselves for a few operations, as
most either already acquire their own wakeref or imply a wakeref. In
particular, it is i915_gem_set_wedged() that needed us to present it
with a wakeref, which is incongruous with its "use anywhere" ability.

Suggested-by: "Yokoyama, Caz" <caz.yokoyama@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Yokoyama, Caz" <caz.yokoyama@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318095204.9913-7-chris@chris-wilson.co.uk