drm/i915: Drop kerneldoc markup from non-kerneldoc enum drrs_refresh_rate_type

DRRS is not yet kerneldoc despite the allusion prior to enum
drrs_refresh_rate_type. Drop the '**' to avoid the warnings from
make htmldocs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161231112012.29263-4-chris@chris-wilson.co.uk