[AGPGART] Move [un]map_page_into_agp into asm/agp.h

Remove an arch-dependent hunk in favor of #define-ing the respective bits in
asm-<arch>/agp.h (allowing easier overriding in para-virtualized environments).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Dave Jones <davej@redhat.com>