drm/radeon: wire up a pci shutdown callback

Normally on shutdown or reboot we don't care about necessarily
making sure the hw is in a good state because the system is about
to be powered down or reset.  However, after a shutdown or reboot
in a VM, it's best to tear down the hw properly otherwise there
can be problems with the next VM use.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>