video: tegra: avoid null deref on nvdps read

When reading nvdps sysfs file, check mode to avoid a null dereference.

Bug 1032235
Bug 1283024

Change-Id: I19b3b5b3de6743cdcc9e3a846a4ba102de681ad3
Signed-off-by: Haley Teng <hteng@nvidia.com>