drm/tegra: Switch over to vmemdup_user()

This patch fixes the following Coccinelle warning:

drivers/gpu/drm/tegra/submit.c:173: WARNING opportunity for vmemdup_user

Use vmemdup_user() rather than duplicating its implementation.
This is a little bit restricted to reduce false positives.

Signed-off-by: Qing Wang <wangqing@vivo.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>