iris: Fix a bug with Tigerlake's auto-fast-clearing mechanism and blorp_copy()

While working on blitter support, I recently came across a bug where using blorp_copy() to perform a blit via util_try_blit_via_copy_region would cause Piglit's copy-pixels to fail. This was apparently #3732 (closed) which was known for a long time now, but now easy to reproduce.

Here's a patch series to fix that, made after lots of conversations with @nchery and @currojerez today. @jekstrand likely has an opinion as well.

I don't like this series, because it's relying on undocumented hardware behavior. But it seems like the least awful thing to do, and if this really works, it gives us an out for handling other rendering-to-texture-view issues which could hit the same problem.