drm/i915: Update debugfs describe_obj() to show fault-mappable

The current meaning of whether an object has a GGTT vma is very
ill-defined (and note we don't check for any partials either), it just
means that at some point it was in the GGTT but it may not be now. The
information we really care about here is whether it is taking up
precious mappable aperture space. This is the obj->fault_mappable flag.
We have a redundant long form reprinting of this information, so remove
that in favour of the compact flag.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161012114827.17031-2-chris@chris-wilson.co.uk