drm/omap: dsi: untangle ulps ops from enable/disable

Create a custom function pointer for ULPS and use it instead of
reusing disable/enable functions for ULPS mode switch. This allows
us to use the common disable/enable functions pointers for DSI.

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/20201215104657.802264-28-tomi.valkeinen@ti.com