drm/i915: Hold rpm wakeref for debugfs/i915_drop_caches_set

Since we peek into HW state and poke around, it behoves us to acquire a
runtime pm wakeref beforehand.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108343
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108364


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181015115856.18590-1-chris@chris-wilson.co.uk