staging: panel: Remove magic numbers in LCD commands

Get rid of magic numbers in LCD commands and replace them with defined
values, so that it's more obvious that the commands are doing.

Signed-off-by: Mariusz Gorski <marius.gorski@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>