The "data-mapping" property may not be the best way to describe the
interface between panels and display interfaces.
Drop use of in the panel-simple driver, so we have time to find
the right way to describe this interface.

Fixes: 4a1d0dbc ("drm/panel: simple: add panel-dpi support")
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200314153047.2486-3-sam@ravnborg.org