media: subdev: rename subdev-state alloc & free

v4l2_subdev_alloc_state() and v4l2_subdev_free_state() are not supposed
to be used by the drivers. However, we do have a few drivers that use
those at the moment, so we need to expose these functions for the time
being.

Prefix the functions with __ to mark the functions as internal.

At the same time, rename them to v4l2_subdev_state_alloc and
v4l2_subdev_state_free to match the style used for other functions like
video_device_alloc() and media_request_alloc().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>