drm/omap: Reverse direction of DSS device (dis)connect operations

The omapdrm and omapdss drivers are architectured based on display
pipelines made of multiple components handled from sink (display) to
source (DSS output). This is incompatible with the DRM bridge and panel
APIs that handle components from source to sink.

To reconcile the omapdrm and omapdss drivers with the DRM bridge and
panel model, we need to reverse the direction of the DSS device
operations. Start with the connect and disconnect operations.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>