Allow VRAM presence only in optional bank.
control: Implement banks.
The BAR is 64MB.
The little-endian/big-endian ranges repeat every 16MB.
An endian range can be addressed in d128 (wide) mode or d64 mode.
In d128 mode, there is a 4MB range followed by an undefined 4MB range. bytes 0..7 belong to the standard bank and bytes 8..15 belong to the optional bank.
In d64 mode:
- 2MB mirror ranges at 0MB and 2MB (both have the same read/write behavor: write to both banks or read from the standard bank.
- 2MB standard bank rage at 4MB.
- 2MB optional bank range at 6MB.
