eventdev: promote adapter functions as stable

eventdev: promote adapter functions as stable

Promote the adapter functions and rte_event_port_unlinks_in_progress()
as stable as it's been added for a while now and multiple drivers and
test application like test-eventdev has been tested using the adapter APIs.

Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Abhinandan Gujjar <abhinandan.gujjar@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>