drm/sun4i: tcon-top: Remove mux configuration at probe time

Now that R40 TCON migrated to runtime mux configuration, old code can be
removed.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180710203511.18454-18-jernej.skrabec@siol.net