There is a possibility of having just one DMA window available with
a limited capacity which the existing code does not handle that well.
If the window is big enough for the system RAM but less than
MAX_PHYSMEM_BITS (which we want when persistent memory is present),
we create 1:1 window and leave persistent memory without DMA.

This disables 1:1 mapping entirely if there is persistent memory and
either:
- the huge DMA window does not cover the entire address space;
- the default DMA window is removed.

This relies on reverted 54fc3c68
("powerpc/pseries/ddw: Extend upper limit for huge DMA window for persistent memory")
to return the actual amount RAM in ddw_memory_hotplug_max() (posted
separately).
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211108040320.3857636-4-aik@ozlabs.ru