powerpc/kernel: Avoid preemption check in iommu_range_alloc()

Replace the __this_cpu_read() with raw_cpu_read() in
iommu_range_alloc(). Otherwise we get a warning about using
__this_cpu_read() in preemptible code:

  BUG: using __this_cpu_read() in preemptible
  caller is iommu_range_alloc+0xa8/0x3d0

Preemption doesn't need to be disabled since according to the comment
any CPU can safely use any IOMMU pool.

Signed-off-by: Victor Aoqui <victora@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>