drm/i915/gt: Check for arbitration after writing start seqno

On the off chance that we need to arbitrate before launching the
payload, perform the check after we signal the request is ready to
start. Assuming instantaneous processing of the CS event, the request
will then be treated as having started when we make the decisions as to
how to process that CS event.

v2: More commentary about the users of i915_request_started() as a
reminder about why we are marking the initial breadcrumb.

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/20210112100759.32698-1-chris@chris-wilson.co.uk