drm/i915/selftests: Provide a mock GPU reset routine

For those mock tests that may wish to pretend triggering a GPU reset and
processing the cleanup.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927211749.2181-3-chris@chris-wilson.co.uk