drm/tegra: hdmi: Fix build warnings

These seem to show up when building for architectures other than ARM,
which I guess will never happen. The reason why the kbuild test bot ran
into these was a missing dependency which has hence been fixed. Still it
doesn't hurt to fix them anyway.

Reported-by: kbuild test bot <fengguang.wu@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>