drm/i915: Drop rpm wakeref on error in debugfs/i915_drop_caches_set

Use single exit point to drop rpm wakeref in case of an error.

Fixes: 9d3eb2c3

 ("drm/i915: Hold rpm wakeref for debugfs/i915_drop_caches_set")
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018092025.24076-1-joonas.lahtinen@linux.intel.com