drm/omap: dsi: display_disable cleanup

We can drop dsi_display_disable() which just calls
_dsi_display_disable(), and rename _dsi_display_disable() to
dsi_display_disable().

The WARN_ON(!dsi_bus_is_locked(dsi)) in dsi_display_disable is extra and
can be dropped, as _dsi_display_disable() has the same WARN_ON().

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/20201215104657.802264-78-tomi.valkeinen@ti.com