drm/omap: dsi: remove ulps support

ULPS is a niche power-saving feature which only really affects command
mode panels showing a static picture. I know the ULPS code used to work
very long time ago, but I could not get it working with the current
driver. As the ULPS code is not trivial and includes delayed work (so
lots of chances for race issues), and just keeping DSI video and command
mode panels working has been challenging enough even without ULPS, lets
remove ULPS support.

When the DSI driver works reliably for command and video mode displays,
someone interested can work on ULPS and add it back if the power saving
is substantial enough.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-83-tomi.valkeinen@ti.com