drm/i915/selftests: add local workqueue for SW fence selftest

Instead of using a global workqueue for the SW fence selftest,
allocate a separate one temporarily only while running the test.

Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/313f4a713053c2b4896ced5b0e9ff456eb85fe57.1686231190.git.jani.nikula@intel.com