[AGPGART] Remove unnecessary flushes when inserting and removing pages.

This patch is to speed up flipping of pages in and out of the AGP aperture as
needed by the new drm memory manager.

A number of global cache flushes are removed as well as some PCI posting flushes.
The following guidelines have been used:

1) Memory that is only mapped uncached and that has been subject to a global
cache flush after the mapping was changed to uncached does not need any more
cache flushes. Neither before binding to the aperture nor after unbinding.

2) Only do one PCI posting flush after a sequence of writes modifying page
entries in the GATT.

Signed-off-by: Thomas Hellstrom <thomas@tungstengraphics.com>
Signed-off-by: Dave Jones <davej@redhat.com>