drm/amdxcp: fix Makefile to build amdxcp module

After drm conduct amdgpu Makefile, amdgpu.ko has been created
and "amdgpu-y +=" in amdxcp Makefile isn't used.
So modify amdgpu-y to amdxcp-y and build amdxcp module.

Signed-off-by: Bob Zhou <bob.zhou@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>