drm/i915: Only do gtt cleanup in vma_unbind for the global vma

Otherwise we end up tearing down fences when e.g. the client quits
way too early. Might or might not fix a fence pin_count BUG Ville has
reported.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>