drm/usb: move usb support into a separate module

In order to satisfy all the various Kconfig options between
USB and DRM, we need to split the USB code out into a separate module
and export symbols to it.

This fixes build problems in -next reported by sfr.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 7990283..d3aaeb6 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -986,6 +986,7 @@
 
 	return 0;
 }
+EXPORT_SYMBOL(drm_mode_group_init_legacy_group);
 
 /**
  * drm_mode_config_cleanup - free up DRM mode_config info