gallium/radeon: always return the last SDMA fence on SDMA flush if needed

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/gallium/drivers/radeon/r600_pipe_common.c

 	struct r600_common_context *rctx = (struct r600_common_context *)ctx;
 	struct radeon_winsys_cs *cs = rctx->rings.dma.cs;
 
-	if (!cs->cdw) {
-		return;
-	}
+	if (!cs->cdw)
+		goto done;
 
 	rctx->rings.dma.flushing = true;
-	rctx->ws->cs_flush(cs, flags, fence, 0);
+	rctx->ws->cs_flush(cs, flags, &rctx->last_sdma_fence, 0);
 	rctx->rings.dma.flushing = false;
+done:
+	if (fence)
+		rctx->ws->fence_reference(fence, rctx->last_sdma_fence);
 }
 
 static enum pipe_reset_status r600_get_reset_status(struct pipe_context *ctx)
 	if (rctx->allocator_so_filled_size) {
 		u_suballocator_destroy(rctx->allocator_so_filled_size);
 	}
+	rctx->ws->fence_reference(&rctx->last_sdma_fence, NULL);
 }
 
 void r600_context_add_resource_size(struct pipe_context *ctx, struct pipe_resource *r)

src/gallium/drivers/radeon/r600_pipe_common.h

 	enum radeon_family		family;
 	enum chip_class			chip_class;
 	struct r600_rings		rings;
+	struct pipe_fence_handle	*last_sdma_fence;
 	unsigned			initial_gfx_cs_size;
 	unsigned			gpu_reset_counter;