drm/amdgpu: Add copy_pte_num_dw member in amdgpu_vm_pte_funcs

Use it to replace the hard coded value in amdgpu_vm_bo_update_mapping().

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