drm/radeon/kms: add 6xx/7xx CS parser for async DMA (v2)

Allows us to use the DMA ring from userspace.
DMA doesn't have a good NOP packet in which to embed the
reloc idx, so userspace has to add a reloc for each
buffer used and order them to match the command stream.

v2: fix address bounds checking, reloc indexing

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