staging: fbtft: Use standard MIPI DCS command defines for hx8340bn

This patch makes use of the standard MIPI Display Command Set to remove
some of the magic constants found in source code.

Signed-off-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>