drm/atmel-hlcdc: Replace the panel usage with drm_panel_bridge.

This cuts 135 lines of boilerplate, at the cost of losing the
filtering of get_modes() using atmel_hlcdc_dc_mode_valid().  The
atomic check will still check that we don't set an invalid mode,
though.

Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511183128.25085-7-eric@anholt.net