drm/i915/selftests: Finish pending mock requests on cancellation.

Flush all the pending requests from the mock engine when they are
cancelled.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200930163253.2789-2-chris@chris-wilson.co.uk