drm/amdgpu: access stolen VRAM directly on KV/KB (v2)

We don't need to use the PCI BAR on APUs. This allows us to access
the full VRAM directly without being limited by the BAR size.

v2: squash in 64bit shift fix

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