drm/i915: GEM on PAE has problems - disable it for now.

On PAE systems, GEM allocates pages using shmem, and passes these
pages to be bound into AGP, however the AGP interfaces + the x86
set_memory interfaces all take unsigned long not dma_addr_t.

The initial fix for this was a mess, so we need to do this correctly
for 2.6.29.

Signed-off-by: Dave Airlie <airlied@redhat.com>