drm/i915: Store number of active engines in device info

Until now code was calling hweight32 to figure out the
number from device_info->ring_mask at runtime. Instead
we can cache it at engine init time and use directly.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1470842530-35854-1-git-send-email-tvrtko.ursulin@linux.intel.com