iommu/arm-smmu: Check smmu->impl pointer before dereferencing

Commit 0d97174a ("iommu/arm-smmu: Implement ->probe_finalize()")
added a new optional ->probe_finalize callback to 'struct arm_smmu_impl'
but neglected to check that 'smmu->impl' is present prior to checking
if the new callback is present.

Add the missing check, which avoids dereferencing NULL when probing an
SMMU which doesn't require any implementation-specific callbacks:

  | Unable to handle kernel NULL pointer dereference at virtual address
  | 0000000000000070
  |
  | Call trace:
  |   arm_smmu_probe_finalize+0x14/0x48
  |   of_iommu_configure+0xe4/0x1b8
  |   of_dma_configure_id+0xf8/0x2d8
  |   pci_dma_configure+0x44/0x88
  |   really_probe+0xc0/0x3c0

Fixes: 0d97174a ("iommu/arm-smmu: Implement ->probe_finalize()")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Will Deacon <will@kernel.org>