drm/tegra: dpaux: Remove unneeded variable

Fix the following coccicheck REVIEW:
./drivers/gpu/drm/tegra/dpaux.c:282:13-16 REVIEW Unneeded variable

Reported-by: Zeal Robot <zealci@zte.com.cm>
Signed-off-by: chiminghao <chi.minghao@zte.com.cn>
Signed-off-by: Thierry Reding <treding@nvidia.com>