drm/amdgpu: Use dynamic IP offset for register access on SOC15

Update the register access macros and functions to take into
account the new dynamic IP base offsets.

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>