omap: zero is a valid fd number, treat it as such

Equivalent to the previous patch.

Cc: Rob Clark <robdclark@gmail.com>
Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>