drm/i915/selftests: Flush the context worker

When cleaning up the mock device, remember to flush the context worker
to free the residual GEM contexts before shutting down the device.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/802


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191230165821.3840449-1-chris@chris-wilson.co.uk