s390/thp: respect page protection in pmd_none() and pmd_present()

Similar to pte_none() and pte_present(), the pmd functions should also
respect page protection of huge pages, especially PROT_NONE.
This patch also simplifies massage_pgprot_pmd() by adding new definitions
for huge page protection.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>