drm/radeon: remove vm_manager start/suspend

Just restore the page table instead. Addressing three
problem with this change:

1. Calling vm_manager_suspend in the suspend path is
   problematic cause it wants to wait for the VM use
   to end, which in case of a lockup never happens.

2. In case of a locked up memory controller
   unbinding the VM seems to make it even more
   unstable, creating an unrecoverable lockup
   in the end.

3. If we want to backup/restore the leftover ring
   content we must not unbind VMs in between.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>