drm/tegra: Remove bogus tegra_framebuffer structure

Tegra uses the CMA FB helpers so framebuffers passed to the driver need
to use the corresponding functions to access the underlying GEM objects.

This used to work because struct tegra_framebuffer was sufficiently
similar to struct drm_fb_cma but that isn't guaranteed to stay that way.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>