drm: omapdrm: dss: Use supported outputs instead of display types

The dss driver uses the supported display types to check whether the DSS
has SDI, VENC or HDMI outputs. We can instead use the supported outputs
the provide the same information. This removes the last use of the
supported display types, that we can then remove as well.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>