drm/i915/gem: Refactor tests on obj->ops->flags

We repeat obj->ops->flags in our object checks, so pull that into its
own little helper for clarity.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002123014.1545-1-chris@chris-wilson.co.uk