backends: Update idle time after opening the lid

When handling lid state, we used to update the idle time right after opening the lid. This behavior changed in commit 14b6c878 due to a typo/thinko, "if (lid_is_closed)" used to be an early return condition before updating idle time, now it only updates in that case.

Restore the original behavior, since this idle time update is key in having gsd-power light up the display again, this presumably fixes situations that required extra "light up" hints after suspend. What it does surely fix is "ninja test" in g-s-d against recent mutter, since the behavioral change induced a test timeout there.