drm/msm/gem: Fix error return on fence id alloc fail

This was a typo, we didn't actually want to return zero.

Fixes: a61acbbe

 ("drm/msm: Track "seqno" fences by idr")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/491145/
Link: https://lore.kernel.org/r/20220624184528.4036837-1-robdclark@gmail.com