x86/amd-iommu: Remove command buffer resetting logic

The logic to reset the command buffer caused more problems
than it actually helped. The logic jumped in when the IOMMU
hardware doesn't execute commands anymore but the reasons
for this are usually not fixed by just resetting the command
buffer. So the code can be removed to reduce complexity.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>