drm: Fix the kernel doc warning for drm_link_status

This fixes the kernel doc warning that was introduced in
the 'commit 40ee6fbe

 ("drm: Add a new connector
atomic property for link status")'. Description has
been added for the enum values.

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1488379510-15059-1-git-send-email-manasi.d.navare@intel.com