drm/sun4i: Create minimal multipliers and dividers

The various outputs the TCON can provide have different constraints on the
dotclock divider. Let's make them configurable by the various mode_set
functions.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/92ff5881c8f8674056d34458b2f264cd48d4e136.1513854122.git-series.maxime.ripard@free-electrons.com