media: tegra-video: Fix compilation warning of unused variable

vi_pattern_strings is used only when CONFIG_VIDEO_TEGRA_TPG is
enabled and V4L2 control operations currently are used only in
TPG mode.

So when tegra-video is build for non TPG, warnings of unused
variable is reported for v4l2 control operation variable.

This patch fixes it.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>