PM / devfreq: event: remove duplicate devfreq_event_get_drvdata()

there define two devfreq_event_get_drvdata() function in devfreq-event.h
when disable CONFIG_PM_DEVFREQ_EVENT, it will lead to build fail. So
remove devfreq_event_get_drvdata() function.

Fixes: f262f28c ("PM / devfreq: event: Add devfreq_event class")
Cc: stable@vger.kernel.org
Signed-off-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>