[media] v4l: vsp1: bru: Fix minimum input pixel size

According to the spec, the minimum input pixel size for BRU is 1px,
not 4px.

Signed-off-by: Takanari Hayama <taki@igel.co.jp>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>