drm/omap: Merge omap_dss_device type and output_type fields

The omap_dss_device type and output_type fields differ mostly for
historical reasons. The output_type field is required for all devices
but the display at the end of the pipeline, and must be set to
OMAP_DISPLAY_TYPE_NONE for the latter. The type field is required for
all devices but the internal encoder, for which it is ignored.

The only reason why the output_type field must be set to
OMAP_DISPLAY_TYPE_NONE for the display at the end of the pipeline is to
identify omap_dss_device instances corresponding to displays. This is
not documented and confusing.

Clean the code by adding a new display field to the omap_dss_device
structure to identify displays, and merge the type and output_type
fields.

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