drm/tegra: Make legacy fbdev support optional

A lot of the modern userspace is capable of working without the legacy
fbdev support. kmscon can be used as a replacement for the framebuffer
console, and KMS X drivers create their own framebuffers.

Most people don't have a system where all of this works yet, though, so
leave support enabled by default.

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