iris: Avoid extra resolves and some stencil-specific code for blits
A perf CI run (nchery#43) against the latest scheduled run (mesa_master@b9fccafed61) reports:
- Changes for some synthetic benchmarks, but they don't actually call iris_blit.
- FPS bump of 1.1% (+/-0.06%) for Counter-Strike GO on TGL (presumably true as a L4D2 trace is helped).