drm/amdgpu: increase max xgmi physical node for aldebaran

aldebaran supports up to 16 xgmi physical nodes.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>