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;