drm: rcar-du: Rework clock configuration based on hardware limits

The DU channels that have a display PLL (DPLL) can only use external
clock sources, and don't have an internal clock divider (with the
exception of H3 ES1.x where the post-divider is present and needs to be
used as a workaround for a DPLL silicon issue).

Rework the clock configuration to take this into account, avoiding
selection of non-existing clock sources or usage of a missing
post-divider.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>