MMU is disabled.
This expands some earlier logic and avoids a number of potential problems:
1. The CPU may not be able to access the framebuffer in real mode (real
mode does not necessarily encompass all available memory, especially
under a hypervisor).
2. Real mode accesses generally assume cacheability, so it might not
even have worked.
3. The difference in cacheability between real mode and later (and
potentially earlier) points in the boot with the MMU on may cause
ERAT parity problems, resulting in a machine check.
This fixes real-mode (usefdt=1) early boot on the G5 iMac, which was
previously broken as a result of issue #3. Late boot will require some
other fixups.
4f75b93007