v4l: vsp1: Add support for multiple DRM pipelines

The R-Car H3 ES2.0 VSP-DL instance has two LIF entities and can drive
two display pipelines at the same time. Refactor the VSP DRM code to
support that by introducing a vsp_drm_pipeline object that models one
display pipeline.

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