drm/i915/execlists: Skip redundant resubmission

If we unwind the active requests, and on resubmission discover that we
intend to preempt the active contexts with themselves, simply skip the
ELSP submission.

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/20191003210100.22250-1-chris@chris-wilson.co.uk