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 de1add36


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