drm: atmel-hlcdc: rework the output code to support drm bridges

The current output code only supports connection to drm panels.
First simplify the drm panel code, and then add support for external drm
bridges.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>