powerpc/mm: Update the WIMG check during H_ENTER

Support for 0 value for memeory coherence is optional and with ppc64
we can always enable memory coherence. Linux kernel did that during
the development of 4.7 kernel. But that resulted in failure in Qemu
in H_ENTER hcall due to below check. The mentioned change was reverted
in the kernel and kernel right now enable memory coherence only if
cache inhibited is not set. Nevertheless update qemu WIMG flag check
to cover the case where we enable memory coherence along with cache
inhibited flag.

In order to handle older and newer kernel version consider both Cache
inhibitted and (cache inhibitted | memory conference) as valid values
for wimg flags.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>