drm/i915: Fix kerneldoc warnings for intel_ringbuffer

drm/i915: Fix kerneldoc warnings for intel_ringbuffer

drivers/gpu/drm/i915/intel_ringbuffer.c:179: warning: No description found for parameter 'req'
drivers/gpu/drm/i915/intel_ringbuffer.c:741: warning: No description found for parameter 'req'
drivers/gpu/drm/i915/intel_ringbuffer.c:741: warning: No description found for parameter 'cs'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208111220.32293-1-chris@chris-wilson.co.uk