staging: media: tegra-video: move private struct declaration to C file

struct tegra_vi_graph_entity is an internal implementation detail of the VI
module. Move its declaration from vi.h to vi.c.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>