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

Abeit quite unlikely to get hit by this bug here, let just fix it.

v2: Correct conditional (do not call ioctl(DRM_IOCTL_PRIME_HANDLE_TO_FD)
when we already have the fd).
v3: Fix kgsl_pipe.c, suggested by Thierry.

Cc: freedreno@lists.freedesktop.org
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>