drm: rcar-du: crtc: Support external DSI dot clock

On platforms with an external clock, both the group and crtc must be
handled accordingly to correctly pass through the external clock and
configure the DU to use the external rate.

The CRTC support was missed while adding the DSI support on the r8a779a0
which led to the output clocks being incorrectly determined.

Ensure that when a CRTC is routed through the DSI encoder, the external
clock is used without any further divider being applied.

Fixes: b291fdcf ("drm: rcar-du: Add r8a779a0 device support")
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>