drm/amdgpu: calculate MCUMC_ADDRT0 per asic's UMC offset

Hardcoded offset is not friendly. And another benifit of this
patch is to keep read and write access to this register be
consistent with other similar UMC regsiters  in this file.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>