drm: rcar-du: Fix external clock error checks

The rcar-du driver supports probe deferral for external clocks, but
implements it badly by checking the wrong pointer due to a bad copy and
paste. Fix it.

While at it, reject invalid clocks outright for DU channels that have a
display PLL, as the external clock is mandatory in that case. This
avoids a WARN_ON() at runtime.

Fixes: 1b30dbde ("drm: rcar-du: Add support for external pixel clock")
Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>