drm/i915: Do not re-calculate num_rings locally

Since bb8f0f5a ("drm/i915: Split intel_engine allocation
and initialisation") intel_info->num_rings is set early in the
load sequence and so available to be used direclty in the 2nd
load phase.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170616130339.23015-1-tvrtko.ursulin@linux.intel.com