drm/i915: Fix up map and fenceable for VMA

formerly: "drm/i915: Create VMAs (part 3.5) - map and fenceable
tracking"

The map_and_fenceable tracking is per object. GTT mapping, and fences
only apply to global GTT. As such,  object operations which are not
performed on the global GTT should not effect mappable or fenceable
characteristics.

Functionally, this commit could very well be squashed in to a previous
patch which updated object operations to take a VM argument.  This
commit is split out because it's a bit tricky (or at least it was for
me).

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Drop the bogus hunk in i915_vma_unbind as discussed with
Ben.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>