drm/nouveau: use post-decrement in error handling

We need to use post-decrement to get the dma_map_page undone also for
i==0, and to avoid some very unpleasant behaviour if dma_map_page
failed already at i==0.
Signed-off-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: NBen Skeggs <bskeggs@redhat.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1520,7 +1520,7 @@ nouveau_ttm_tt_populate(struct ttm_tt *ttm)
 				    DMA_BIDIRECTIONAL);
 
 		if (dma_mapping_error(pdev, addr)) {
-			while (--i) {
+			while (i--) {
 				dma_unmap_page(pdev, ttm_dma->dma_address[i],
 					       PAGE_SIZE, DMA_BIDIRECTIONAL);
 				ttm_dma->dma_address[i] = 0;