drm/stm: ltdc: add mask for lxcr register

The purpose of this mask is to simplify writing to the lxcr
register and not to forget any fields.

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240712131423.98405-1-yannick.fertre@foss.st.com


Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>