drm/i915/guc: Decouple GuC engine id from ring id

Previously GuC uses ring id as engine id because of same definition.
But this is not true since this commit:

commit de1add3605
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Fri Jan 15 15:12:50 2016 +0000

    drm/i915: Decouple execbuf uAPI from internal implementation

Added GuC engine id into GuC interface to decouple it from ring id used
by driver.

v2: Keep ring name print out in debugfs; using for_each_ring() where
    possible to keep driver consistent. (Chris W.)

Signed-off-by: Alex Dai <yu.dai@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453579094-29860-1-git-send-email-yu.dai@intel.com

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -158,6 +158,7 @@ struct  intel_engine_cs {
 #define I915_NUM_RINGS 5
 #define _VCS(n) (VCS + (n))
 	unsigned int exec_id;
+	unsigned int guc_id;
 	u32		mmio_base;
 	struct		drm_device *dev;
 	struct intel_ringbuffer *buffer;