drm/stm: ltdc: add non-alpha color formats

ltdc supports natively some color formats with alpha (like
ARGB8888, ARGB1555, ARGB4444...). Related non-alpha formats are
supported too (ARGB8888->XRGB8888, ARGB4444->XRGB4444...) by
adjusting ltdc blending factors.

Note: Wayland/Weston requests by default the non-alpha XRGB8888
color format.

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/20180201104243.20726-2-philippe.cornu@st.com