drm/omap: dsi: Reverse direction of the DSS device enable/disable operations

Complete the direction reversal of the DSS device enable/disable
operations started by commit 19b4200d ("drm/omap: Reverse direction
of the DSS device enable/disable operations").

This effectively drops the requirement of calling DSS specific
code from the DSI panel driver moving it a bit further to a
standard drm_panel driver.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-31-tomi.valkeinen@ti.com