video: fbdev: radeon: constify pci_device_id.

pci_device_id are not supposed to change at runtime. All functions
working with pci_device_id provided by <linux/pci.h> work with
const pci_device_id. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
  19709	   5024	     32	  24765	   60bd	video/fbdev/aty/radeon_base.o

File size after adding 'const':
   text	   data	    bss	    dec	    hex	filename
  22893	   1840	     32	  24765	   60bd	video/fbdev/aty/radeon_base.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Antonino Daplas <adaplas@gmail.com>
Cc: Maik Broemme <mbroemme@libmpq.org>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>