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>