staging: fbtft: Make some s16 arrays const

Using const reduces data size.

$ size drivers/staging/fbtft/built-in.o*
   text	   data	    bss	    dec	    hex	filename
  89909	  41584	   2928	 134421	  20d15	drivers/staging/fbtft/built-in.o.new
  88053	  43392	   2928	 134373	  20ce5	drivers/staging/fbtft/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>