x86, AMD IOMMU: remove unneeded initializations from command buffer allocation

This patch removes an unneeded initialization from the alloc_command_buffer
function and replaces a memset with __GFP_ZERO.

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>