drm/amdgpu: use xarray for storing pasid in vm

Replace idr with xarray as we actually need hash functionality.
Cleanup code related to vm pasid by adding helper function.

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