[AGPGART] Allow drm-populated agp memory types

This patch allows drm to populate an agpgart structure with pages of its own.
It's needed for the new drm memory manager which dynamically flips pages in and out of AGP.

The patch modifies the generic functions as well as the intel agp driver. The intel drm driver is
currently the only one supporting the new memory manager.

Other agp drivers may need some minor fixing up once they have a corresponding memory manager enabled drm driver.

AGP memory types >= AGP_USER_TYPES are not populated by the agpgart driver, but the drm is expected
to do that, as well as taking care of cache- and tlb flushing when needed.

It's not possible to request these types from user space using agpgart ioctls.

The Intel driver also gets a new memory type for pages that can be bound cached to the intel GTT.

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