drm/stm: ltdc: fix warnings in ltdc_plane_create()

"make C=1" returns 2 warnings in ltdc_plane_create()
("Using plain integer as NULL pointer"). This patch
fixes them.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180419132804.8317-1-philippe.cornu@st.com