drm/i915/perf: Annotate i915_perf.wakeref for keneldoc

drivers/gpu/drm/i915/i915_drv.h:1375: warning: Function parameter or member 'wakeref' not described in 'i915_perf_stream'

Reported-by:  <kbuild-all@01.org>
Fixes: 6619c007 ("drm/i915/perf: Track the rpm wakeref")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190115102505.4843-1-chris@chris-wilson.co.uk