drm: radeon: radeon_ttm: constify ttm_place structures.

ttm_place are not supposed to change at runtime. All functions
working with ttm_place provided by <drm/ttm/ttm_placement.h> work
with const ttm_place. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   9235	    344	    136	   9715	   25f3 drivers/gpu/drm/radeon/radeon_ttm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9267	    312	    136	   9715	   25f3 drivers/gpu/drm/radeon/radeon_ttm.o
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -178,7 +178,7 @@ static int radeon_init_mem_type(struct ttm_bo_device *bdev, uint32_t type,
 static void radeon_evict_flags(struct ttm_buffer_object *bo,
 				struct ttm_placement *placement)
 {
-	static struct ttm_place placements = {
+	static const struct ttm_place placements = {
 		.fpfn = 0,
 		.lpfn = 0,
 		.flags = TTM_PL_MASK_CACHING | TTM_PL_FLAG_SYSTEM