drm/i915/gt: Be paranoid and reset the GPU before release

Just in the very unlikely case we have not stopped the GPU before we
return the pages being used by the GPU to the system, force a reset.

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/20200117180309.3249427-1-chris@chris-wilson.co.uk