media: v4l: vsp1: Print the correct blending unit name in debug messages

The DRM pipelines can use either the BRU or the BRS for blending. Make
sure the right name is used in debugging messages to avoid confusion.

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