[media] v4l: vsp1: Document calling context of vsp1_pipeline_propagate_alpha()

The function can only be called from a s_stream handler as it requires a
valid display list context (due to calling vsp1_uds_set_alpha() which
writes to module registers). Document the requirement.

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