drm/radeon: use cached memory when evicting for vram on non agp

Force the use of cached memory when evicting from vram on non agp
hardware. Also force write combine on agp hw. This is to insure
the minimum cache type change when allocating memory and improving
memory eviction especialy on pci/pcie hw.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>