drm/tegra: vic: Fix implicit function declaration warning

When CONFIG_IOMMU_API isn't set the following warnings pops up:

drivers/gpu/drm/tegra/vic.c: In function ‘vic_boot’:
drivers/gpu/drm/tegra/vic.c:110:31: 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(vic->dev);
                               ^~~~~~~~~~~~~~~~~~~~
                               iommu_fwspec_free
drivers/gpu/drm/tegra/vic.c:110:31: warning: initialization of ‘struct iommu_fwspec *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
drivers/gpu/drm/tegra/vic.c:117:19: error: ‘struct iommu_fwspec’ has no member named ‘num_ids’
   if (spec && spec->num_ids > 0) {
                   ^~
drivers/gpu/drm/tegra/vic.c:118:16: error: ‘struct iommu_fwspec’ has no member named ‘ids’
    value = spec->ids[0] & 0xffff;
                ^~

Rework so that its inside a '#ifdef CONFIG_IOMMU_API' block.

Fixes: f3779cb1 ("drm/tegra: vic: Support stream ID register programming")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>