drm/omap: dss: Create and use omapdss_device_is_registered()

The omapdss_component_is_loaded() function test whether a component is
loaded by checking whether it is present in the displays list or the
outputs list. Simplify the implementation by checking for the component
in the global omap_dss_device list.

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>