drm/omap: Check omap_dss_device type based on the output_type field

Various functions that need to differentiate between omap_dss_device
instances corresponding to displays and to internal encoders use the
omap_dss_device.driver field, which is only set for display instances.
This gets in the way of the omap_dss_device operations refactoring.
Replace that with a check based on the output_type field which is set
for all omap_dss_device instances but displays.

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>