drm/i915: Inline check required for object syncing prior to execbuf

This trims a little overhead from the common case of not needing to
synchronize between rings.

v2: execlists is special and likes to duplicate code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>