drm/i915: Forcibly flush unwanted requests in drop-caches

Add a mode to debugfs/drop-caches to flush unwanted requests off the GPU
(by wedging the device and resetting). This is very useful if a test
terminated leaving a long queue of hanging batches that would ordinarily
require a round trip through hangcheck for each.

It reduces the inter-test operation to just a write into drop-caches to
reset driver/GPU state between tests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903083337.13134-5-chris@chris-wilson.co.uk