drm: sun4i: calculate proper DCLK rate for DSI

In DSI mode, TCON0's data clock is required to run at 1/4 the per-lane
bit rate.

Signed-off-by: Roman Beranek <me@crly.cz>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230505052110.67514-5-me@crly.cz