drm/tegra: Enable the hotplug interrupt only when necessary

The hotplug handling needs access to the DRM device, which only appears
at ->init() time. Disable interrupts up until that time. Similarly, when
an output is removed, disable the hotplug interrupt again because the
DRM device (and with it the hotplug infrastructure) is going away.

Also make sure to only access the DRM device if it's available. Given
the above change for the hotplug interrupt this should really never
happen, but the extra check doesn't hurt either.

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