drm/i915: Simplify i915_gem_obj_ggtt_bound_view

Can use the new vma->is_gtt to simplify the check and
get rid of the local variables.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461240286-25968-1-git-send-email-tvrtko.ursulin@linux.intel.com


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>