display: Change DPMS init and exit fucntion name

display: Change DPMS init and exit fucntion name

For consistent naming rule with other DPMS function,
change the names.

init_dpms -> dpms_init
exit_dpms -> dpms_exit
add_timer_for_init_dpms -> add_timer_for_dpms_init
delayed_init_done_dpms -> delayed_dpms_init_done

Change-Id: I1de2c9741526238d33c03e815f2ff73fa2a13f9f
Signed-off-by: TaeminYeom <taemin.yeom@samsung.com>