drm/stm: ltdc: remove non-alpha color formats on layer 2 for older hw

Hw older versions support non-alpha color formats derived
from native alpha color formats only on the primary layer.
For instance, RG16 native format without alpha works fine
on 2nd layer but XR24 (derived color format from AR24)
does not work on 2nd layer.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180201104243.20726-3-philippe.cornu@st.com