drm/sun4i: tcon: Switch mux on only for composite

Even though that mux is undocumented, it seems like it needs to be set to 1
when using composite, and 0 when using HDMI.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>