drm/sti: add debugfs fps_show/fps_get mechanism for planes

Display fps on demand for each used plane:
cat /sys/kernel/debug/dri/0/fps_get
Display fps in live in the console for each used plane:
echo 255 > /sys/kernel/debug/dri/0/fps_show

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>