drm/i915: disallow gem ums init ioctl for kms

This ioctl used in a kms driver is only useful to create massive
havoc.

v2: Bail out with -ENODEV as suggested by Chris Wilson.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -140,6 +140,9 @@ i915_gem_init_ioctl(struct drm_device *dev, void *data,
 {
 	struct drm_i915_gem_init *args = data;
 
+	if (drm_core_check_feature(dev, DRIVER_MODESET))
+		return -ENODEV;
+
 	if (args->gtt_start >= args->gtt_end ||
 	    (args->gtt_end | args->gtt_start) & (PAGE_SIZE - 1))
 		return -EINVAL;