gpu: host1x: Fix compile error when IOMMU API is not available

In case the IOMMU API is not available compiling host1x fails with
the following error:

  In file included from drivers/gpu/host1x/hw/host1x06.c:27:
  drivers/gpu/host1x/hw/channel_hw.c: In function ‘host1x_channel_set_streamid’:
  drivers/gpu/host1x/hw/channel_hw.c:118:30: error: implicit declaration of function
    ‘dev_iommu_fwspec_get’; did you mean ‘iommu_fwspec_free’?  [-Werror=implicit-function-declaration]
  struct iommu_fwspec *spec = dev_iommu_fwspec_get(channel->dev->parent);
                              ^~~~~~~~~~~~~~~~~~~~
                              iommu_fwspec_free

Fixes: de5469c2

 ("gpu: host1x: Program the channel stream ID")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>