Only one request may be on the dma interface at a time,

Only one request may be on the dma interface at a time,
so add some states to indicate that the dma read/write
hasn't issued yet, and add some support to reissue the
request when the interface becomes unblocked.

--HG--
extra : convert_revision : 448695a8eb2e9f98554769f3165df1f796adf44a