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: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_modes.h

@@ -138,7 +138,6 @@ struct drm_display_mode {
 	int crtc_vtotal;
 
 	/* Driver private mode info */
-	int private_size;
 	int *private;
 	int private_flags;