drm/omap: dss: Allow looking up any device by port

The omap_dss_find_output_by_port() function looks up an omap_dss_device
by port from the list of devices registered as outputs. In preparation
for looking up sinks in addition to sources, allow the function to look
up any registered device. Rename it to omap_dss_find_device_by_port() to
match its new purpose.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>