drm/tegra: dc: Remove unused variables

Remove unused variables from tegra_crtc_update_memory_bandwidth().

Fixes: 04d5d5df ("drm/tegra: dc: Support memory bandwidth management")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>