drm/i915: Fix missed needs_dmar setting

From Ben's AGP dependence removal change, "needs_dmar" flag has not
been properly setup for new chips using new GTT init function. This
one adds missed setting of that flag to make sure we do pci mappings
with IOMMU enabled.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>