[media] v4l: Remove experimental tag from certain API elements

Remove experimantal tag from the following API elements:
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY buffer type.
V4L2_CAP_VIDEO_OUTPUT_OVERLAY capability flag.
VIDIOC_ENUM_FRAMESIZES IOCTL.
VIDIOC_ENUM_FRAMEINTERVALS IOCTL.
VIDIOC_G_ENC_INDEX IOCTL.
VIDIOC_ENCODER_CMD and VIDIOC_TRY_ENCODER_CMD IOCTLs.
VIDIOC_DECODER_CMD and VIDIOC_TRY_DECODER_CMD IOCTLs.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>