drm/sun4i: tcon: Add variant callback for TCON output muxing

Different SoCs have different muxing options and values for the TCON
outputs. Instead of stuffing every possibility in sun4i_tcon_set_mux(),
add a callback pointer to sun4i_tcon_quirks that each TCON variant
can use to provide muxing support.

The current muxing options in sun4i_tcon_set_mux() for sun5i-a13 are
moved to a new sun5i-specific callback function.

Since the new callback replaces what the .has_unknown_mux field in
tcon quirks did in the past, the field is removed.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010032008.682-2-wens@csie.org