Enter display off state after dimming

The logic inherited from N9 went to lpm_on state if low power mode
is enabled. This is not desirable because lpm display states require
locking of ui, which in turn cancels grace period that allows users
to get back to previously open application.

[mce] Enter display off state after dimming. Fixes JB#19192
Merge pull request #161 from spiiroin/lpm_blanking

Enter display off state after dimming