In super_block_uvrd(),if is_cost_valid == 0, all return parameters,
i.e. rate, distortion, skippable, and sse, are reset.
So, should not call txfm_rd_in_plane() if is_cost_valid == 0.
Also, the bug causes av1_xform_quant() to see invalid diff signal
since av1_subtract_plane() is not called in super_block_uvrd().

Change-Id: Iaa06061e2e9aa8876b4611a54f4ae6b8d499332b