drm/amdgpu: SDMA update use unlocked iterator

SDMA update page table may be called from unlocked context, this
generate below warning. Use unlocked iterator to handle this case.

WARNING: CPU: 0 PID: 1475 at
drivers/dma-buf/dma-resv.c:483 dma_resv_iter_next
Call Trace:
 dma_resv_iter_first+0x43/0xa0
 amdgpu_vm_sdma_update+0x69/0x2d0 [amdgpu]
 amdgpu_vm_ptes_update+0x29c/0x870 [amdgpu]
 amdgpu_vm_update_range+0x2f6/0x6c0 [amdgpu]
 svm_range_unmap_from_gpus+0x115/0x300 [amdgpu]
 svm_range_cpu_invalidate_pagetables+0x510/0x5e0 [amdgpu]
 __mmu_notifier_invalidate_range_start+0x1d3/0x230
 unmap_vmas+0x140/0x150
 unmap_region+0xa8/0x110

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