drm/omap: dsi: do ULPS in host driver

Move ULPS handling into the DSI host controller, so that we
no longer need a custom API for the DSI client.

Note that for now ULPS is always disabled.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-29-tomi.valkeinen@ti.com