powerpc: Remove unnecessary condition when sanity-checking WIMG bits

It is okay for both _PAGE_GUARDED and _PAGE_COHERENT (G and M) to be set
in the same pte.  In fact, even if that were not the case, there doesn't
seem to be any place where G is set without also setting I (_PAGE_NO_CACHE),
so the test for I is sufficient as a condition to clear _PAGE_COHERENT
when filling the hash table.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>