drm: rcar-du: Link HDMI encoder with bridge

The conversion of the rcar-du driver from the I2C slave encoder to the
DRM bridge API left the HDMI encoder's bridge pointer NULL, preventing
the bridge from being handled automatically by the DRM core. Fix it.

Fixes: 1d926114 ("drm: rcar-du: Remove i2c slave encoder interface for hdmi encoder")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>