Fixed the following sparse warning using proper NULL instead of '0': - mm_gk20a.c:1301: warning: Using plain integer as NULL pointer Bug 200067946 Change-Id: Idd84f541711682bf097bb474049d523a5bb01ae2 Signed-off-by: Amit Sharma (SW-TEGRA) <amisharma@nvidia.com> Reviewed-on: http://git-master/r/682242 Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com> Reviewed-by: Automatic_Commit_Validation_User Reviewed-by: Sachin Nikam <snikam@nvidia.com>