drm: fix drm_drv_uses_atomic_modeset on non modesetting drivers.

vgem seems to oops on the intel CI due to the vgem debugfs init
hitting this path now.

Check if we have mode_config funcs before checking one.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180918062018.24942-1-airlied@gmail.com

include/drm/drm_drv.h

@@ -675,7 +675,7 @@ static inline bool drm_core_check_feature(struct drm_device *dev, int feature)
 static inline bool drm_drv_uses_atomic_modeset(struct drm_device *dev)
 {
 	return drm_core_check_feature(dev, DRIVER_ATOMIC) ||
-		dev->mode_config.funcs->atomic_commit != NULL;
+		(dev->mode_config.funcs && dev->mode_config.funcs->atomic_commit != NULL);
 }