Avoid writing to the frame buffer in early boot on PowerPC if the CPU's
MMU is disabled.
This expands some earlier logic and avoids a number of potential problems:
- 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).
- Real mode accesses generally assume cacheability, so it might not even have worked.
- 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