dimdisplay: only dim the screen at configured dim time
The current logic is completely broken: 1. It disables the screen at `m_dimOnIdleTime` even though disabling the screen is handled elsewhere with a separate timeout, and the logic here has prefix "Dim*" so has nothing to do with disabling it. 2. It does not follow the timeout configured by a user (that is the `m_dimOnIdleTime`) and instead dims the screen at completely arbitrary 50% and 75% periods of the configured time. Let's get rid of all that and only do exactly what the user asked: dim the screen at `m_dimOnIdleTime`. BUG: 304696 FIXED-IN: 5.27.8