hwmon: amoled-ecm: add support to re-enable ECM dynamically

When ECM is enabled, disable ECM if display goes blank, and reenable
ECM once display restores to unblank. This helps in keeping ECM HW
enabled only when display is active.

Meanwhile, update the logic a little bit on calculating the average
current. When any of the accumulated values is invalid, just ignore
the calculation instead of disabling the ECM.

Change-Id: Ibfe973df4dabd1b1cd48927ce567a094da6382a3
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>