drm/tegra: hdmi: Make sure clock is enabled before dumping registers

The debugfs register dumping function did not enable the HDMI clock.
This led to a possible system hang when reading the debugfs entry
while no HDMI cable was connected to the system. This patch makes
sure that the clock is enabled during the read.

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