drm: of: Add drm_of_lvds_get_data_mapping

Add helper function to convert DT "data-mapping" property string value
into media bus format value, and deduplicate the code in panel-lvds.c
and lvds-codec.c .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
To: dri-devel@lists.freedesktop.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211012224252.29185-1-marex@denx.de