drm/amdgpu: Don't use shadow BO for compute context

Compute contexts cannot keep going after a GPU reset. Currently the process
must terminate. In the future a process may be able recreate its context
from scratch. Either way, there is no need to restore the GPUVM page table
from shadow BOs.

Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>