drm/omap: Pass both output and display omap_dss_device to encoder init

The drm_encoder implementation requires access to the omap_dss_device
corresponding to the display, which is passed to its initialization
function and stored internally. Clean up of the HDMI mode and infoframe
handling will require access to the output omap_dss_device. To prepare
for that, pass it to the encoder initialization function and store it
internally as well.

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>