i915_gem_drain_workqueue() is not used on any hot paths. Un-unline it.

Replace the do-while with a for loop while at it.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2c89e7e0a3528caf7ba9ffa29b2bb9f13f2357d1.1662390010.git.jani.nikula@intel.com