Fix PRT handling on gfx9

v2: unify PRT bit for all ASICs
v3: move PRT flag checking in amdgpu_vm_bo_split_mapping()
Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: David Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>