gallium/swr: spin-lock performance improvement

gallium/swr: spin-lock performance improvement

Currently, the worker threads are very aggresively
polling for new tasks. If the work is not constantly
fed into the pipeline (which is a case for most of
interactive applications), this creates unnecessary
memory pressure and is using CPU cycles that
could otherwise be used by the applications.
The change implements simple back off mechanism
to help with this problem

Change by Tomasz Pyra (tomasz.pyra@intel.com)

Reviewed-by: Alok Hota <alok.hota@intel.com>
Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4226>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4226>