We're nearly out of dirty bits, and some patches pending review on
GitLab no longer apply due to that.  Make room for them by splitting
off shader stage-specific bits into a separate stage_dirty mask.

An alternative would be to split compute-related bits into a separate
mask, but that would prevent the '<< stage' indexing done in various
parts of the driver from working.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <!5279>