drm/i915/breadcrumbs: Use booleans for intel_breadcrumbs_busy()

Since commit 9b6586ae ("drm/i915: Keep a global seqno per-engine")
converted intel_breadcrumbs_busy() to reporting a single boolean, we
need only compute a boolean internally (and not needlessly compute the
flag).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170315210726.12095-1-chris@chris-wilson.co.uk