drm/amdgpu: Expose *_setup_vm_pt_regs for kfd to use

kfd has the same need to set the VM page table base register, so expose
them for kfd to use for better maintainability.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>