drm/radeon: Fix EEH during kexec

During kexec some adapters hit an EEH since they are not properly
shut down in the radeon_pci_shutdown() function. Adding
radeon_suspend_kms() fixes this issue.
Enabled only on PPC because this patch causes issues on some other
boards.

Signed-off-by: Kyle Mahlkuch <kmahlkuc@linux.vnet.ibm.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>