Revert "[media] v4l: vsp1: Supply frames to the DU continuously"

This reverts commit 3299ba5c ("[media] v4l: vsp1: Supply frames to
the DU continuously")

The DU output mode does not rely on frames being supplied on the WPF as
its pipeline is supplied from DRM. For the upcoming WPF writeback
functionality, we will choose to enable writeback mode if there is an
output buffer, or disable it (leaving the existing display pipeline
unharmed) otherwise.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>