vp9_postproc: disable VP9D_DEMACROBLOCK w/uv_width < 8

`VP9D_DEMACROBLOCK` also uses `vp9_deblock()`, which requires a minimum
width of 8.

Follow up to:
 1963b530e vp9_postproc: disable VP9D_DEBLOCK w/uv_width < 8

Bug: 499602810
Change-Id: I9a46239ce068ecf72d795f087d601e721bf18feb