css_vgpu.c: fix warnings found by sparse tool Bug 200088648 Change-Id: I4d9d33c6438be8fa8f2ba9cb0b2f786c46b7c9ce Signed-off-by: Peter Daifuku <pdaifuku@nvidia.com> Reviewed-on: http://git-master/r/1234605 Reviewed-by: Aingara Paramakuru <aparamakuru@nvidia.com> GVS: Gerrit_Virtual_Submit Reviewed-by: Amit Sharma (SW-TEGRA) <amisharma@nvidia.com> Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>