drm/i915: Tidy common test_bit probing of i915_request->fence.flags

A repeated pattern is to test the signaled bit of our
request->fence.flags. Make this an inline to shorten a few lines and
remove unnecessary line continuations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190121222117.23305-20-chris@chris-wilson.co.uk