powerpc/mm: Fix handling of _PAGE_COHERENT in BAT setup code

_PAGE_COHERENT is now always set in _PAGE_RAM resp. PAGE_KERNEL.
Thus it has to be masked out, if the BAT mapping should be non
cacheable or CPU_FTR_NEED_COHERENT is not set.

This will work on normal SMP setups because we force-set
CPU_FTR_NEED_COHERENT as part of CPU_FTR_COMMON on SMP.

Signed-off-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>