drm/sun4i: tcon: Move out the tcon0 common setup

Some channel0 setup has to be done, no matter what the output interface is
(RGB, CPU, LVDS). Move that code into a common function in order to avoid
duplication.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/183100/