agp/intel-agp: Set dma_mask for capable chipsets before agp_add_bridge()

We should set this before calling agp_add_bridge() so that it's done
before we map the scratch page too.

This should probably fix the regression reported as k.o. bug #14627.

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