Move graphic-related coalesced MMIO flushes to affected device models

This is conceptually cleaner and will allow us to drop the nographic
timer. Moreover, it will be mandatory to fully exploit future per-device
coalesced MMIO rings.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
