video: fbdev: aty: make const arrays static, reduces object code size

Don't populate the const arrays ragepro_tbl and ragexl_tbl on the stack,
instead make them static.  Makes the object code smaller by over 380 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  41089	  10592	    768	  52449	   cce1 atyfb_base.o

After:
   text	   data	    bss	    dec	    hex	filename
  40544	  10752	    768	  52064	   cb60	atyfb_base.o

(gcc version 7.2.0, x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>