drm/tegra: Unconditionally select IOMMU_IOVA

Currently configurations can be generated where IOMMU_SUPPORT is
disabled but IOMMU_IOVA is built as a module and DRM_TEGRA as built-in.
In such a case, the symbols guarded by IOMMU_IOVA will not be available
when linking the Tegra DRM driver and cause a linking failure.

Simplify this by unconditionally selecting IOMMU_IOVA, which makes sure
that it will be forced to =y if DRM_TEGRA=y. Technically we can now get
IOMMU_IOVA code built-in even if we don't use it (Tegra DRM only uses it
when IOMMU_SUPPORT is also enabled), but such configuration are of a
mostly academic nature. In all practical configurations we want IOMMU
support anyway.

Signed-off-by: Thierry Reding <treding@nvidia.com>