drm/tegra: Removed fb from struct tegra_fbdev

Fbdev's struct fb_helper stores a pointer to the framebuffer. Remove
struct tegra_fbdev.fb, which contains the same value. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>