drm/i915: Flush context free work on cleanup

Throw in a flush_work() to specifically flush the context cleanup work
before the module is unloaded.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112248
Fixes: a4e7ccda ("drm/i915: Move context management under GEM")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112150051.1603-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -776,6 +776,7 @@ int i915_gem_init_contexts(struct drm_i915_private *i915)
 void i915_gem_driver_release__contexts(struct drm_i915_private *i915)
 {
 	destroy_kernel_context(&i915->kernel_context);
+	flush_work(&i915->gem.contexts.free_work);
 }
 
 static int context_idr_cleanup(int id, void *p, void *data)