drm/atmel-hlcdc: Fix panel registration

Attach the panel-bridge created by drm_panel_bridge_add() to the
parallel RGB encoder.

Fixes: 96160a80 ("drm/atmel-hlcdc: Replace the panel usage with drm_panel_bridge.")
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1496748866-17165-1-git-send-email-boris.brezillon@free-electrons.com