drm/amd/display: use the proper fb offset for DM

This fixes DMCU initialization in APU GPU passthrough.  The
DMCU needs the GPU physical address, not the CPU physical
address.  This ends up working out on bare metal because
we always use the physical address, but doesn't work in
passthrough because the addresses are different.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>