drm/sun4i: Fix tcon channel 0 comment about backporch = backporch + hsync

The backporch programmed into the tcon registers is actually the
backporch + hsync length from the display timings, as indicated in
the interface timing diagrams found in the user manual of the A31
and A33 SoCs.

The comments for channel 0 mistakenly describe the discrepancy as
TCON backporch = frontporch + hsync.

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