drm/tilcdc: Add support for drm panels

Add support for drm panels to tilcdc. Adding the support on top of the
existing bridge support needs only couple of lines of code when using
using the drm panel bridge helpers.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>