drm/vmwgfx: Improve on IOMMU detection

instead of relying on intel_iommu_enabled, use the fact that the
dma_map_ops::map_page != dma_direct_map_page.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>