drm/omap: Fix port lookup for SDI output

drm/omap: Fix port lookup for SDI output

When refactoring port lookup for DSS outputs, commit d17eb4537a7e
("drm/omap: Factor out common init/cleanup code for output devices")
incorrectly hardcoded usage of DT port 0. This breaks operation for SDI
(which uses the DT port 1) and DPI outputs other than DPI0 (which are
not used in mainline DT sources).

Fix this by using the port number from the output omap_dss_device
of_ports field.

Fixes: d17eb4537a7e ("drm/omap: Factor out common init/cleanup code for output devices")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821183226.13784-1-laurent.pinchart@ideasonboard.com
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>