incorporate write back support for processors not having a write through
cache.
The current fb_devconfig structure now really holds the device's
configuration and the softc really only holds the attachment information.
This used to be mixed giving rise to weird stuctures and cross references.
The number of vertical syncs before the video memory writeback is triggered
is configurable ... default is to wait for 5 Vsync .. aprox minumum 10
times a second, but more likely in the order of 12,5 times a second. When
printing is in progress no write back is performed... only after the
waiting time. The reasoning behind this is that as long as the screen is
printed too the cache will be purged of dirty data anyway due to the
processing and new screen memory useage.
2b488a7d9f