drm: Fix render node numbering regression from control node removal.

drm_minor_alloc() does multiplication on this enum, so the removal
ended up moving render nodes down from 128 base to 64.  This caused
Mesa's surfaceless backend to be unable to open the render nodes,
since it was still looking up at 128.

v2: Add a comment warning the next person.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 0d49f303e8 ("drm: remove all control node code")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180509001425.12574-1-eric@anholt.net

include/drm/drm_file.h

@@ -47,8 +47,12 @@ struct device;
  * header include loops we need it here for now.
  */
 
+/* Note that the order of this enum is ABI (it determines
+ * /dev/dri/renderD* numbers).
+ */
 enum drm_minor_type {
 	DRM_MINOR_PRIMARY,
+	DRM_MINOR_CONTROL,
 	DRM_MINOR_RENDER,
 };