drm/stm: ltdc: add pinctrl for DPI encoder mode

The implementation of functions encoder_enable and encoder_disable
make possible to control the pinctrl according to the encoder type.
The pinctrl must be activated only if the encoder type is DPI.
This helps to move the DPI-related pinctrl configuration from
all the panel or bridge to the LTDC dt node.

Reviewed-by: Philippe Cornu <philippe.cornu@st.com>

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1567761708-31777-1-git-send-email-yannick.fertre@st.com