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


(cherry picked from commit 5f00cac9)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>