drm/ttm: remove priority hard code when initializing ttm bo

Then priority could be set before initialization.
By default, it requires to kzalloc ttm bo. In fact, we always do so.

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: David Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>