data-mapping may not be the best way to describe the
data format used between panels and display interface.

Drop it from the panel-dpi binding so we do not start to rely on it.
We can then work out how to best describe this mapping and when
we know it, we can add it to this binding.

Fixes: fa102240 ("dt-bindings: display: add data-mapping to panel-dpi")
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Rob Herring <robh@kernel.org>
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
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200314153047.2486-2-sam@ravnborg.org