drm: remove drm_display_mode->private_size

It' unused and there's also not really any way to make it work with
the current code. So better rip it out.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm_modes.h b/include/drm/drm_modes.h
index 995c34d..2dbbf99 100644
--- a/include/drm/drm_modes.h
+++ b/include/drm/drm_modes.h
@@ -138,7 +138,6 @@
 	int crtc_vtotal;
 
 	/* Driver private mode info */
-	int private_size;
 	int *private;
 	int private_flags;