NOMMU: Stub out vm_get_page_prot() if there's no MMU

Stub out vm_get_page_prot() if there's no MMU.

This was added by commit 804af2cf ("[AGPGART] remove private page
protection map") and is used in commit c07fbfd1

 ("fbmem: VM_IO set,
but not propagated") in the fbmem video driver, but the function doesn't
exist on NOMMU, resulting in an undefined symbol at link time.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>