anv: Decouple flush_descriptor_sets() from pipeline struct

anv: Decouple flush_descriptor_sets() from pipeline struct

Explicitly pass the active stages and the array (and size) of shaders
to be processed.  This will make easy to store only the shaders needed
for each pipeline.

The active stages can be identified by a non-NULL shader in the
shaders array, so stop using it and keep track of the flushed stages
as iteration happens.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4040>