drm/radeon: Constify struct pci_device_id

'struct pci_device_id' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  11984	  28672	     44	  40700	   9efc	drivers/gpu/drm/radeon/radeon_drv.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  40000	    664	     44	  40708	   9f04	drivers/gpu/drm/radeon/radeon_drv.o

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>