drm/amdgpu: Dynamic initialize IP base offset

The base offsets of the IP blocks may change across
asics even though the relative register offsets
are the same for an IP.  Handle this dynamically.

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>