drm/sun4i: tcon: Fix tcon channel 1 backporch calculation

It seems like what's called a backporch in the datasheet is actually the
backporch plus the sync period. Fix that in our driver.

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