staging: fbtft: make const array gamma_par_mask static

Don't populate array gamma_par_mask on the stack but instead make it
static.  Makes the object code smaller by 148 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   2993	   1104	      0	   4097	   1001	drivers/staging/fbtft/fb_st7789v.o

After:
   text	   data	    bss	    dec	    hex	filename
   2757	   1192	      0	   3949	    f6d	drivers/staging/fbtft/fb_st7789v.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>