staging: media: tegra-video: move default format to soc-specific data

The tegra_default_format in vi.c is specific to Tegra210 CSI.

In preparation for adding Tegra20 VIP support, move the default format to a
new field in the soc-specific `struct tegra_vi_soc`. Instead of an entire
format struct, only store a pointer to an item in the existing format
array.

No functional changes. The format pointed to is the same that used to be in
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>