drm/i915: Avoid early GPU idling due to already pending idle work

Atm, in case an idle work handler is already pending but haven't yet
started to run, retiring a new request will not extend the active period
as required, rather simply leaves the pending idle work to be scheduled
at the original expiration time. This may lead to idling the GPU too
early. Fix this by using the delayed-work scheduler alternative which
makes sure the handler's expiration time is extended in this case.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Requested-by: Chris Wilson <chris@chris-wilson.co.uk>
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/1478510405-11799-1-git-send-email-imre.deak@intel.com