drm/omap: Move most omap_dss_driver operations to omap_dss_device_ops

omap_dss_device instances have two ops structures, omap_dss_driver and
omap_dss_device_ops. The former is used for devices at the end of the
pipeline (a.k.a. display devices), and the latter for intermediate
devices.

Having two sets of operations isn't convenient as code that iterates
over omap_dss_device instances need to take them both into account.
There's currently a reasonably small amount of such code, but more will
be introduced to move the driver away from recursive operations. To
simplify current and future code, move all operations that are not
specific to the display device to the omap_dss_device_ops.

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>