x86 gart: replace to_pages macro with iommu_num_pages

This patch removes the to_pages macro from x86 GART code and calls the generic
iommu_num_pages function instead.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>