drm/stm: ltdc: fix duplicated arguments

Fix COMPILE_TEST build issue detected with the
rule: "duplicated argument to & or |"

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1495445421-20846-1-git-send-email-philippe.cornu@st.com