drm/i915/selftests: Attach a stub pm_domain

Supply a pm_domain and its ops for our mock GEM device so that
device runtime pm doesn't complain even though we only want to mark it
permanently active!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170718173028.31207-1-chris@chris-wilson.co.uk


Tested-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>