The reason we mapped them bidirectionally was that not doing
so had caused IOMMU exceptions, due to the fact that the HW
writes back into the command. Now that the first part of the
command including the write-back part is always in the first
buffer, we don't need to map the remaining buffer(s) bidi
and can get rid of the special-casing for commands.

This is a requisite patch for another one to fix DMA mapping.
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>