[media] vsp1: declar vsp1_pipeline_stopped() as static

drivers/media/platform/vsp1/vsp1_video.c:517:6: warning: no previous prototype for 'vsp1_pipeline_stopped' [-Wmissing-prototypes]
 bool vsp1_pipeline_stopped(struct vsp1_pipeline *pipe)
      ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>