drm/tegra: Make gather_bo_ops static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

./drivers/gpu/drm/tegra/submit.c:136:28: warning: symbol 'gather_bo_ops'
was not declared. Should it be static?

Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>