drm/i915: Remove the GEM idle worker

Nothing inside the idle worker now requires struct_mutex, so we can
remove the indirection of using our own worker.

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/20191004134015.13204-9-chris@chris-wilson.co.uk