drm/i915: fix possible RPM ref leaking during RPS disabling

In

commit c6df39b5
Author: Imre Deak <imre.deak@intel.com>
Date:   Mon Apr 14 20:24:29 2014 +0300

    drm/i915: get a runtime PM ref for the deferred GT powersave enabling

I added an RPM get-ref when enabling RPS from a deferred work, but forgot
to add the corresponding put-ref when canceling the work. This may leave
RPM disabled.

Note that the race is real since we run the rps enabling with a
delayed work item after resume, so leaves enough time (in contrived
examples) to fit a quick autoresum in.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Robert Beckett <robert.beckett@intel.com>
Testecase: igt/pm_rpm/system-suspend
[danvet: Mention testcase and add note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>