drm/sun4i: support TCONs without channel 1

Some Allwinner SoCs, such as the A33, have a variation of the TCON that
doesn't have a second channel (or it is not wired to anything).

Make sure we can handle that case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>