r600g: implement buffer copying using CP DMA for R7xx, Evergreen, Cayman

r600g: implement buffer copying using CP DMA for R7xx, Evergreen, Cayman

R6xx doesn't work - the issue seems to be with flushing (sometimes
the destination buffer contains garbage). There are no hangs, so we're good.

R7xx doesn't seem to have any alignment restriction despite our initial
thinking. Everything just works.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>