drm/amd/display: Add NULL test for 'timing generator' in 'dcn21_set_pipe()'

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Add NULL test for 'timing generator' in 'dcn21_set_pipe()' In "u32 otg_inst = pipe_ctx->stream_res.tg->inst;" pipe_ctx->stream_res.tg could be NULL, it is relying on the caller to ensure the tg is not NULL.