drm/omap: dsi: implement check timings

Implement check timings, which will check if it's possible to
configure the clocks for the provided mode using the same code
as the set_config() hook.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-35-tomi.valkeinen@ti.com