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>

drivers/gpu/drm/amd/amdxcp/Makefile

@@ -20,6 +20,6 @@
 # OTHER DEALINGS IN THE SOFTWARE.
 #
 
-amdgpu-y := amdgpu_xcp_drv.o
+amdxcp-y := amdgpu_xcp_drv.o
 
-obj-$(CONFIG_DRM_AMDGPU) += amdgpu_xcp_drv.o
+obj-$(CONFIG_DRM_AMDGPU) += amdxcp.o