drm/stm: ltdc: add user update info in plane print state

This patch adds the user update information in
frames-per-second into the drm debugfs plane state.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180407213503.30932-1-philippe.cornu@st.com