drm/amdgpu: Unmap all MMIO mappings

Access to those must be prevented post pci_remove

v6: Drop BOs list, unampping VRAM BAR is enough.
v8:
Add condition of xgmi.connected_to_cpu to MTTR
handling and remove MTTR handling from the old place.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210517193105.491461-1-andrey.grodzovsky@amd.com