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:

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.