drm/sun4i: tcon-top: Add helpers for mux switching

We want to be able to set TCON TOP muxes at runtime. Add helpers for
that.

Old, static configuration of muxes at probe time is preserved for now.
It will be removed when R40 TCON starts using them.

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/20180710203511.18454-12-jernej.skrabec@siol.net