drm/stm: dsi: Constify phy ops structure

Constify dw_mipi_dsi_stm_phy_ops as these ops are not supposed
to change at runtime.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500552357-29487-8-git-send-email-philippe.cornu@st.com