drm/omap: dss: Make omap_dss_get_next_device() more generic

Despite its name, the omap_dss_get_next_device() function operates on
display devices only. Make it more generic by allowing operation on all
devices, with a parameter to specify the device type.

While at it rename the function to omapdss_device_get_next() to match
the naming of the other functions operating on struct omap_dss_device.

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>