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: NChristian König <christian.koenig@amd.com>
Reviewed-by: NMichel Dänzer <mdaenzer@redhat.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>