powerpc: Remove superfluous WARN_ON() from dma-noncoherent.c

I can't tell why this WARN_ON exists, and there's no comment
explaining it.  Whether the pmd is present or not, pte_alloc_kernel()
seems to handle both cases.

Booting a 440 kernel with 64K PAGE_SIZE triggers the warning, but boot
successfully completes and I see no problems beyond that.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>