[media] v4l: vsp1: Cleanup video nodes at removal time

Video nodes created and initialized in the RPF and WPF init code paths
are never unregistered, and the related resources (videobuf alloc
context and media entity) never released.

Fix this by storing a pointer to the vsp1_video object in vsp1_entity
and calling vsp1_video_cleanup() from vsp1_entity_destroy(). This also
allows simplifying the init error code paths.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>