drm/amdkfd: Move dma unmapping after TLB flush

Otherwise GPU may access the stale mapping and generate IOMMU
IO_PAGE_FAULT.

Move this to inside p->mutex to prevent multiple threads mapping and
unmapping concurrently race condition.

After kfd_mem_dmaunmap_attachment is removed from unmap_bo_from_gpuvm,
kfd_mem_dmaunmap_attachment is called if failed to map to GPUs, and
before free the mem attachment in case failed to unmap from GPUs.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>