drm/amdgpu/gmc: use PCI BARs for APUs in passthrough

If the GPU is passed through to a guest VM, use the PCI
BAR for CPU FB access rather than the physical address of
carve out.  The physical address is not valid in a guest.

v2: Fix HDP handing as suggested by Michel

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>