drm: rcar-du: Move CRTC outputs bitmask to private CRTC state

The rcar_du_crtc outputs field stores a bitmask of the outputs driven by
the CRTC. This changes based on the configuration requested by
userspace, and is used for the sole purpose of configuring the hardware.
The field thus belongs to the CRTC state. Move it to the
rcar_du_crtc_state structure.

As a result the rcar_du_crtc_route_output() function loses most of its
purpose. In order to remove it, move dpad0_source calculation to
rcar_du_atomic_commit_tail(), until the field gets moved to a state
structure. In order to simplify the rcar_du_group_set_routing()
implementation, we also store the DPAD1 source in a new dpad1_source
field which will move to a state structure with dpad0_source.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>