drm/amdgpu: use a macro to define no xcp partition case

~0 as no xcp partition is used in several places, so improve its
definition by a macro for code consistency.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>