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_stub.c b/drivers/gpu/drm/drm_stub.c
index bbd40ea..aa454f8 100644
--- a/drivers/gpu/drm/drm_stub.c
+++ b/drivers/gpu/drm/drm_stub.c
@@ -319,6 +319,7 @@
 	drm_lastclose(dev);
 	return retcode;
 }
+EXPORT_SYMBOL(drm_fill_in_dev);
 
 
 /**
@@ -397,6 +398,7 @@
 	*minor = NULL;
 	return ret;
 }
+EXPORT_SYMBOL(drm_get_minor);
 
 /**
  * Put a secondary minor number.
@@ -428,6 +430,7 @@
 	*minor_p = NULL;
 	return 0;
 }
+EXPORT_SYMBOL(drm_put_minor);
 
 static void drm_unplug_minor(struct drm_minor *minor)
 {