drm/i915: Object w/o backing storage is banned by -ENXIO

-ENXIO should be returned when operations are banned from changing
backing storage of objects without backing storage.

v4:
- update "Reviewed-by". (Joonas)

v3:
- separate this patch from "Introduce GEM proxy" patch-set. (Joonas)

v2:
- update the patch description and subject to just mention objects w/o
  backing storage, instead of "GEM proxy". (Joonas)
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1510555798-21079-1-git-send-email-tina.zhang@intel.comReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114102513.22269-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -1725,7 +1725,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 	 */
 	if (!obj->base.filp) {
 		i915_gem_object_put(obj);
-		return -EINVAL;
+		return -ENXIO;
 	}
 
 	addr = vm_mmap(obj->base.filp, 0, args->size,