drm/i915: Pull seqno started checks together

We have a few instances of checking seqno-1 to see if the HW has started
the request. Pull those together under a helper.

v2: Pull the !seqno assertion higher, as given seqno==1 we may indeed
check to see if we have started using seqno==0.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806112605.20725-1-chris@chris-wilson.co.uk