drm/tegra: Fix error handling cleanup

The DRM driver's ->load() implementation didn't do a good job (no job at
all really) cleaning up on failure. Fix that by undoing any prior setup
when an error occurs. This requires a bit of rework to make it possible
to clean up fbdev midway.

This was tested by injecting errors at various points during the
initialization sequence and verifying that error cleanup didn't crash
and no memory leaked (using kmemleak).

Reported-by: Stéphane Marchesin <marcheu@chromium.org>
Reported-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>