dt-bindings: display: grammar fixes in panel/

Fix a few grammar/editorial issues spotted by Laurent Pinchart.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200314153047.2486-4-sam@ravnborg.org