Attaching to the i915_active barrier is a two stage process, and a flush
is only effective when the barrier is activation. Thus it is possible
for us to see a barrier, and attempt to flush, only for our flush to
have no effect. As such, before attempting to activate signaling on the
fence we need to double check it is a fence!

Fixes: d13a3177 ("drm/i915: Flush idle barriers when waiting")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/1333Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227085723.1961649-1-chris@chris-wilson.co.uk