drm/stm: dsi: add power on/off phy ops

These new physical operations are helpful to power_on/off the dsi
wrapper. If the dsi wrapper is powered in video mode, the display
controller (ltdc) register access will hang when DSI fifos are full.

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1558952499-15418-3-git-send-email-yannick.fertre@st.com