drm/omap: populate DSI platform bus earlier

After the changes from 4.20 the DSI encoder tries to find the
attached panel before populating the DSI bus. If the panel is
not found -EPROBE_DEFER is returned, so the DSI bus is never
populated and the panel never added.

Fix this by populating the DSI bus before searching for the
video sink in dsi_init_output().

Fixes: 27d62452

 ("drm/omap: dss: Acquire next dssdev at probe time")
Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Pavel Machek <pavel@ucw.cz>
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/20181121160916.22017-3-sebastian.reichel@collabora.com