drm/i915: Remove delay for idle_work

The original intent for the delay before running the idle_work was to
provide a hysteresis to avoid ping-ponging the device runtime-pm. Since
then we have also pulled in some memory management and general device
management for parking. But with the inversion of the wakeref handling,
GEM is no longer responsible for the wakeref and by the time we call the
idle_work, the device is asleep. It seems appropriate now to drop the
delay and just run the worker immediately to flush the cached GEM state
before sleeping.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190507121108.18377-2-chris@chris-wilson.co.uk