drm/tegra: Allocate resources at probe time

Since the .init() and .exit() functions are executed whenever the DRM
driver is loaded or unloaded, care must be taken not to use them for
resource allocation. Otherwise deferred probing cannot be used, since
the .init() and .exit() are not run at probe time. Similarly the code
that frees resources must be run at .remove() time. If it is run from
the .exit() function, it can release resources multiple times.

To handle this more consistently, rename the tegra_output_parse_dt()
function to tegra_output_probe() and introduce tegra_output_remove()
which can be used to free output-related resources.

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