drm/tegra: Extract tegra_gem_lookup()

The static function host1x_bo_lookup() in drm.c is also useful
elsewhere. Extract it as tegra_gem_lookup() in gem.c.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>