drm: stm: implement get_scanout_position function

Hardware allow to read the position in scanout buffer so
we can use this information to make wait of vblank more accurate.

Active area bounds (start, end, total height) have already been
computed and written in ltdc registers, read them and get the
current line position to compute vpos value.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Tested-by: Yannick Fertré <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180629130140.16004-1-benjamin.gaignard@linaro.org