drm/radeon: Use ttm_bo_move_null() in radeon_bo_move()

Since ttm_bo_move_null() is exactly the same as ttm_resource_free() +
ttm_bo_assign_mem(), we use ttm_bo_move_null() for the GTT --> SYSTEM
move case too. Then the code is more consistent as the SYSTEM --> GTT
move case.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>