drm/radeon: Implement radeon_pci_shutdown

Currently radeon devices are not properly shutdown during kexec. This
causes a varity of issues, e.g. dpm initialization failures.
Fix this by implementing a radeon_pci_shutdown function, that unloads
the driver cleanly.

Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>