devfreq: Use devfreq_update_stats for gpu governors

This is the recommended way of using the devfreq api from
within governors. The devfreq_update_stats() will utilize the
devfreq.last_status to get the target device status so we no
longer need to declare a local variable to get the target
status.

Change-Id: I5d80e39468e3eb90e55dcb0a813cb846a65ee8a5
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>