media: v4l2-subdev: Turn .[gs]_frame_interval into pad operations

The subdev .[gs]_frame_interval are video operations, but they operate
on pads (and even on streams). Not only is this confusing, it causes
practical issues for drivers as the operations don't receive a subdev
state pointer, requiring manual state handling.

To improve the situation, turn the operations into pad operations, and
extend them to receive a state pointer like other pad operations.

While at it, rename the operations to .[gs]et_frame_interval at the same
time to match the naming scheme of other pad operations. This isn't
strictly necessary, but given that all drivers using those operations
need to be modified, handling the rename separately would generate more
churn for very little gain (if at all).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de> # for imx-media
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com> # for tegra-video
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>