Calls to change_page_attr() need an explicit call to
global_flush_tlb() afterwards.  The AGP code didn't
do this in a number of cases.  This patch makes
map_page_into_agp/unmap_page_from_agp do the calls
themselves, which takes care of most of the problem.

The Intel AGP driver also has some slightly different calls to what
map_page_into_agp() does, as it changes 4 contiguous pages.
Introduce explicit flushes afterwards there too.

Thanks to Alan Cox for pointing this out.
Signed-off-by: Dave Jones <davej@redhat.com>