cpuidle: haltpoll: Take 'idle=' override into account

Currenly haltpoll isn't aware of the 'idle=' override, the priority is
'idle=poll' > haltpoll > 'idle=halt'. When 'idle=poll' is used, cpuidle
driver is bypassed but current_driver in sys still shows 'haltpoll'.

When 'idle=halt' is used, haltpoll takes precedence and makes
'idle=halt' have no effect.

Add a check to prevent the haltpoll driver from loading if 'idle=' is
present.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Co-developed-by: Joao Martins <joao.m.martins@oracle.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>