drm/i915: Simplify i915_gem_obj_to_ggtt_view

Can use vma->is_ggtt to simplify the check and also switch the
BUG_ON to GEM_BUG_ON which is more appropriate for this.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>