drm/i915: Limit page allocations to lowmem (dma32) for i965

Broadwater and Crestline share a limitation that prevent it from
relocating general surface state above 4GiB. The only recourse we have
since any buffer object may be used as a relocation target is then to
limit all object allocations on 965g[m] to DMA32.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>