drm/omap: dsi: do bus locking in host driver

This moves the bus locking into the host driver and unexports
the custom API in preparation for drm_panel support.

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/20201215104657.802264-27-tomi.valkeinen@ti.com