drm/amdgpu: Wait for newly allocated PTs to be idle

When page table are updated by the CPU, synchronize with the
allocation and initialization of newly allocated page tables.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>