etnaviv: drm: Don't miscalculate timeout

The current code overflows (s * 1000000000) for s >= 5 but that is e.g. used in etna_bo_cpu_prep(). Alternatively we could reject larger values in etna_pipe_wait() and reduce the value in etna_bo_cpu_prep().

See https://lore.kernel.org/dri-devel/20200117154726.GA328525@bogon.m.sigxcpu.org/

Freedreno shares has an identical get_abs_timeout() and uses the same timeout in msm_bo_cpu_prep() so it should be affected too.

If that makes sense i'll send a patch for libdrm too.