drm/amdgpu: Handle xgmi device removal.

XGMI hive has some resources allocted on device init which
needs to be deallocated when the device is unregistered.

v2: Remove creation of dedicated wq for XGMI hive reset.
v3: Use the gmc.xgmi.supported flag

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>