drm/ingenic: Properly compute timings when using a 3x8-bit panel

The LCD controller expects timing values in dot-clock ticks, which is 3x
the timing values in pixels when using a 3x8-bit display; but it will
count the display area size in pixels either way. Go figure.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119155559.14112-3-paul@crapouillou.net