Power save and KMS error handling fixes

This MR consists of three fixes:

1. A timeout calculation bug potentially causing us to end up in a semi-busy loop until the page flip FIFO became non-empty.
2. A power saving issue where we in the past would potentially render in a semi-busy loop when power saving. Most likely seldom an issue, as redrawing requires damage, and we usually show the screen shield before power saving meaning no animation.
3. Lack of power save handling in the FIFO-full handling (#509 (closed))