drm/i915: Name the anonymous structs inside i915_ggtt_view

Naming this pair will become useful shortly...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170114002827.31315-1-chris@chris-wilson.co.uk