drm/amdgpu: Correct the IB size of bo update mapping.

The amdgpu_vm_frag_ptes will call amdgpu_vm_update_ptes, and for buffer
object that has shadow buffer, need twice commands.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>