drm/stm: ltdc: add clut mode support

Add the 8-bit clut mode support at crtc level.
Useful for low memory footprint user interfaces but also for
8-bit old games (including color shifting visual effects).
Tested with fbdev FBIOPUTCMAP & drm DRM_IOCTL_MODE_SETGAMMA
ioctls.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1509016666-18927-1-git-send-email-philippe.cornu@st.com