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>

drivers/gpu/drm/radeon/radeon_cs.c

@@ -43,6 +43,7 @@ static int radeon_cs_parser_relocs(struct radeon_cs_parser *p)
 		return 0;
 	}
 	chunk = &p->chunks[p->chunk_relocs_idx];
+	p->dma_reloc_idx = 0;
 	/* FIXME: we assume that each relocs use 4 dwords */
 	p->nrelocs = chunk->length_dw / 4;
 	p->relocs_ptr = kcalloc(p->nrelocs, sizeof(void *), GFP_KERNEL);