drm/i915/error: remove unused gen8_engine_sync_index

Leftover from Gen8 ringbuffer support removal

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305222122.3547-1-daniele.ceraolospurio@intel.com


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>