drm/amdgpu: fix aper_base for APU

[Why]
Wrong fb offset results in dmub f/w errors and white screen.
[drm:dc_dmub_srv_wait_idle [amdgpu]] *ERROR* Error waiting for DMUB idle: status=3

[How]
Read aper_base from mmhub because GC is off by default

v2: use BAR for passthrough (Alex)

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>