drm/ttm: fix broken merge between drm-next and drm-misc-next

drm-next reverted the changes to ttm_tt_create() to do the
NULL check inside the function, but drm-misc-next adds new
users of this approach.

Re-apply the NULL check change inside the function to fix this.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/386628/

drivers/gpu/drm/ttm/ttm_bo.c

@@ -1180,7 +1180,7 @@ int ttm_bo_validate(struct ttm_buffer_object *bo,
 	/*
 	 * We might need to add a TTM.
 	 */
-	if (bo->mem.mem_type == TTM_PL_SYSTEM && bo->ttm == NULL) {
+	if (bo->mem.mem_type == TTM_PL_SYSTEM) {
 		ret = ttm_tt_create(bo, true);
 		if (ret)
 			return ret;

drivers/gpu/drm/ttm/ttm_tt.c

@@ -50,6 +50,9 @@ int ttm_tt_create(struct ttm_buffer_object *bo, bool zero_alloc)
 
 	dma_resv_assert_held(bo->base.resv);
 
+	if (bo->ttm)
+		return 0;
+
 	if (bdev->need_dma32)
 		page_flags |= TTM_PAGE_FLAG_DMA32;
 
@@ -67,7 +70,6 @@ int ttm_tt_create(struct ttm_buffer_object *bo, bool zero_alloc)
 		page_flags |= TTM_PAGE_FLAG_SG;
 		break;
 	default:
-		bo->ttm = NULL;
 		pr_err("Illegal buffer object type\n");
 		return -EINVAL;
 	}