drm/omap: Refactor initialization sequence

The omapdrm driver initialization procedure starts by connecting all
available pipelines, gathering related information (such as output and
display DSS devices, and DT aliases), sorting them by alias, and finally
creates all the DRM/KMS objects.

When using DRM bridges instead of DSS devices, we will need to attach to
the bridges before getting the aliases. As attaching to bridges requires
an encoder object, we have to reorganize the initialization sequence to
create encoders before getting aliases and sorting the pipelines.

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>