agp: use scratch page on memory remove and at GATT creation V4

Convert most AGP chipset to use scratch page as default entries.
This help avoiding GPU querying 0 address and trigger computer
fault. With KMS and memory manager we bind/unbind AGP memory
constantly and it seems that some GPU are still doing AGP
traffic even after GPU report being idle with the memory segment.

Tested (radeon GPU KMS + Xorg + compiz + glxgears + quake3) on :
- SIS 1039:0001 & 1039:0003
- Intel 865 8086:2571

Compile tested for other bridges

V2 enable scratch page on uninorth
V3 fix unbound check in uninorth insert memory (Michel Dänzer)
V4 rebase on top of drm-next branch with the lastest intel AGP
   changeset (stable should use version V3 of the patch)

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>