video: fbdev: radeon: make const array post_divs static, reduces object code size

Don't populate the read-only const array post_divs on the stack,
instead make it static. Makes the object code smaller by 90 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  40231	   8584	    896	  49711	   c22f	radeon_base.o

After:
   text	   data	    bss	    dec	    hex	filename
  39914	   8744	    960	  49618	   c1d2	radeon_base.o

(gcc version 7.2.0, x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>