drm/tegra: Properly cleanup and zero out resources

When the DRM driver is unloaded, all the associated resources must be
cleaned up and zeroed out. This is necessary because of the architecture
of the Tegra DRM driver, where not all subdrivers are unloaded along
with the DRM driver. Therefore device-managed managed won't be freed and
memory cannot be assumed to have been cleared (because it hasn't been
reallocated using kzalloc()) by the time the DRM driver is reloaded. It
is therefore necessary to zero out the structures to prevent strange
errors (such as slab corruptions) from occurring.

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