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/Kconfig b/drivers/gpu/drm/Kconfig
index ed2f690..6b358d1 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -18,6 +18,11 @@
 	  details.  You should also select and configure AGP
 	  (/dev/agpgart) support if it is available for your platform.
 
+config DRM_USB
+	tristate
+	depends on DRM
+	select USB
+
 config DRM_KMS_HELPER
 	tristate
 	depends on DRM