drm/admgpu: Reduce the usage of soc15ip.h

Remove the header where it's not used.

Acked-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>