drm/stm: drv: fix suspend/resume

Without this fix, the system can not go in "suspend" mode
due to an error in drv_suspend function.

Fixes: 35ab6cfb ("drm/stm: support runtime power management")

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1560755897-5002-1-git-send-email-yannick.fertre@st.com