drm/tegra: tegra_fb -> drm_framebuffer

Since tegra_fb is now the same as drm_framebuffer, we can just replace
the type completely.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Thierry Reding <treding@nvidia.com>