intel/blorp: Assert an 8bpp fast clear restriction

intel/blorp: Assert an 8bpp fast clear restriction

We can't do fast clear operations on some LODs of 8bpp surfaces. Add an
assertion to BLORP to protect against drivers attempting to do this.

This assertion was successfully hit with some local modifications to
iris and with the piglit test case, "generatemipmap-base-change format".

Ref: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7301
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Jianxun Zhang <jianxun.zhang@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22857>