drm/amdgpu: reserve fence slot to update page table

drm/amdgpu: reserve fence slot to update page table

Forgot to reserve a fence slot to use sdma to update page table, cause
below kernel BUG backtrace to handle vm retry fault while application is
exiting.

[  133.048143] kernel BUG at /home/yangp/git/compute_staging/kernel/drivers/dma-buf/dma-resv.c:281!
[  133.048487] Workqueue: events amdgpu_irq_handle_ih1 [amdgpu]
[  133.048506] RIP: 0010:dma_resv_add_shared_fence+0x204/0x280
[  133.048672]  amdgpu_vm_sdma_commit+0x134/0x220 [amdgpu]
[  133.048788]  amdgpu_vm_bo_update_range+0x220/0x250 [amdgpu]
[  133.048905]  amdgpu_vm_handle_fault+0x202/0x370 [amdgpu]
[  133.049031]  gmc_v9_0_process_interrupt+0x1ab/0x310 [amdgpu]
[  133.049165]  ? kgd2kfd_interrupt+0x9a/0x180 [amdgpu]
[  133.049289]  ? amdgpu_irq_dispatch+0xb6/0x240 [amdgpu]
[  133.049408]  amdgpu_irq_dispatch+0xb6/0x240 [amdgpu]
[  133.049534]  amdgpu_ih_process+0x9b/0x1c0 [amdgpu]
[  133.049657]  amdgpu_irq_handle_ih1+0x21/0x60 [amdgpu]
[  133.049669]  process_one_work+0x29f/0x640
[  133.049678]  worker_thread+0x39/0x3f0
[  133.049685]  ? process_one_work+0x640/0x640

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
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>