drm/tegra: Do not use ->load() and ->unload() callbacks

The ->load() and ->unload() drivers are midlayers and should be avoided
in modern drivers. Fix this by moving the code into the driver ->probe()
and ->remove() implementations, respectively.

v2: kick out conflicting framebuffers before initializing fbdev
v3: rebase onto drm/tegra/for-next

Tested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>