media: v4l: vsp1: Fix display stalls when requesting too many inputs

Make sure we don't accept more inputs than the hardware can handle. This
is a temporary fix to avoid display stall, we need to instead allocate
the BRU or BRS to display pipelines dynamically based on the number of
planes they each use.

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