drm/sun4i: dsi: Rework a bit the hblk calculation

It turns out that the hblk calculation actually follows a similar pattern
than the other packets. Rework a bit the calculation and add a comment.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d79a21b09847579ce907212a59737af21a729dd0.1549896081.git-series.maxime.ripard@bootlin.com