drm/sun4i: dsi: Fix front vs back porch calculation

Since I always confuse the back and front porches, a few miscalculation
slipped through. Fix them.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/90c2375b8a853cae0dcc135cedb47edbc26168d8.1549896081.git-series.maxime.ripard@bootlin.com