drm/doc: Fix links in drm_property.c

One of the functions was missing () to make the autolinks work,
unfortunately copy-pasted a few times all over.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161123192327.28819-1-daniel.vetter@ffwll.ch