drm/omap: panel-dsi-cm: fix driver

This adds support for get_timings() and check_timings()
to get the driver working and properly initializes the
timing information from DT.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>