[media] vivid: Don't declare .vidioc_overlay twice

Removes the following warnings:

drivers/media/platform/vivid/vivid-core.c:581:2: warning: initialized field overwritten [-Woverride-init]
  .vidioc_overlay   = vidioc_overlay,
  ^
drivers/media/platform/vivid/vivid-core.c:581:2: warning: (near initialization for 'vivid_ioctl_ops.vidioc_overlay') [-Woverride-init]

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>