drm/i915: Rename local struct intel_engine_cs variables

Done by the Coccinelle script below plus a manual
intervention to GEN8_RING_SEMAPHORE_INIT.

@@
expression E;
@@
- struct intel_engine_cs *ring = E;
+ struct intel_engine_cs *engine = E;
<+...
- ring
+ engine
...+>
@@
@@
- struct intel_engine_cs *ring;
+ struct intel_engine_cs *engine;
<+...
- ring
+ engine
...+>

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>