drm/i915: use assert_rpm_wakelock_held instead of opencoding it

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1450203038-5150-7-git-send-email-imre.deak@intel.com