drm/stm: ltdc: add support of flexible pixel formats

This feature allows the generation of any RGB pixel format.
The list of supported formats is no longer linked to the
register LXPFCR_PF, that the reason why a list of drm formats is
defined for each display controller version.

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Acked-by: Philippe Cornu <philippe.cornu@foss.st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@foss.st.com>
Reviewed-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Tested-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215214835.20593-1-yannick.fertre@foss.st.com