iommu/amd: Correctly set flags for handle_mm_fault call

Instead of just checking for a write access, calculate the
flags that are passed to handle_mm_fault() more precisly and
use the pre-defined macros.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-By: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>