drm: amd: amdgpu: 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.

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>