Don't populate the array wtst_xlat on the stack but instead make it
static const. Makes the object code smaller by 89 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  14347	    840	      0	  15187	   3b53	fbdev/matrox/matroxfb_misc.o

After:
   text	   data	    bss	    dec	    hex	filename
  14162	    936	      0	  15098	   3afa	fbdev/matrox/matroxfb_misc.o

(gcc version 9.2.1, amd64)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
[b.zolnierkie: use u8 while at it (suggested by Ville Syrjälä)]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906181114.31414-1-colin.king@canonical.com