drm/omap: dss: Rework output lookup by port node

The omap_dss_find_output_by_port_node() function defined in output.c
looks up an output from its port node. To do so it needs to call helper
functions from dss-of.c to lookup the port parent and the port number.
As omap_dss_find_output_by_port_node() is only called by
omapdss_of_find_source_for_first_ep() from dss-of.c this goes back and
forth between the to source files and isn't very clear.

Simplify the code by passing both the parent and the port number to
omap_dss_find_output_by_port_node() instead of the port node, and rename
the function to omap_dss_find_output_by_port().

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>