drm/i915: Flush the idle-worker for debugfs/i915_drop_caches

drm/i915: Flush the idle-worker for debugfs/i915_drop_caches

After being requested to idle the GPU, flush the idle worker to drop the
residual active state, and any internal object caches.

v2: By popular demand, introduce DROP_IDLE for fine-grained control from
userspace, though it should be used as part of a
	DROP_ACTIVE | DROP_RETIRE | DROP_IDLE | DROP_FREED
sequence.
v3: Convert to BIT() to sell it to Joonas.

References: https://bugs.freedesktop.org/show_bug.cgi?id=102655
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171018121621.10824-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>