nvc0: set ret variable if launch desc allocation failed

nvc0: set ret variable if launch desc allocation failed

Pointed out by gcc

nve4_compute.c: In function 'nve4_launch_grid':
nve4_compute.c:511:7: warning: 'ret' may be used uninitialized in
 this function [-Wmaybe-uninitialized]
    if (ret)
       ^

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Edit by Christoph Bumiller:
Set it to -1 to indicate failure and only when it's actually required.