tu: Fix context faults loading unused descriptor sets

tu: Fix context faults loading unused descriptor sets

The app is allowed to never bind descriptor sets that are statically
unused by the pipeline, which would've caused a context fault since
CP_LOAD_STATE6 would try to load the descriptors that don't exist. Fix
this by not preloading descriptors from unused descriptor sets. We could
do more fine-grained accounting of which descriptors are used, but this
is enough to fix the problem.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5400>