drm/sun4i: tcon: Move the muxing out of the mode set function

The muxing can actually happen on both channels on some SoCs, so it makes
more sense to just move it out of the sun4i_tcon1_mode_set function and
create a separate function that needs to be called by the encoders.

Let's do that and convert the existing drivers.

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>