drm/i915: More renaming of rings to engines

This time using only sed and a few by hand.

v2: Rename also intel_ring_id and intel_ring_initialized.
v3: Fixed typo in intel_ring_initialized.

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/1458126040-33105-1-git-send-email-tvrtko.ursulin@linux.intel.com