drm/sti: fix build warnings in sti_drv.c and sti_vtg.c files

Fix compilation warning introduced by:
commit 0c7ff84f ("drm/sti: remove deprecated legacy vtg slave")
commit 5e60f595

 ("drm/sti: use atomic_helper for commit")

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>