freedreno: ubwc mipmaps
renderdoc trace perf results from a previous revision, which I expect to hold up:
traces/glmark2/texture-texture-filter=mipmap.rdc: +9.1% +/- 2.2%
traces/android/trex.rdc: +8.7% +/- 0.4%
traces/glmark2/desktop-effect=shadow:windows=4.rdc: +4.2% +/- 2.5%