drm/stm: ltdc: move pinctrl to encoder mode set

The pin control must be set to default as soon as possible to
establish a good video link between tv & bridge hdmi
(encoder mode set is call before encoder enable).

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1574850218-13257-1-git-send-email-yannick.fertre@st.com