drm/i915: Update describe_obj

Make it aware of which domain it is bound into GGTT, or PPGTT.

While modifying the function, add a global gtt flag to the object
description. Global is more interesting than aliasing since aliasing is
the default.

v2: Access VMA directly for start/size instead of helpers (Daniel)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>