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.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Tested-by: Marge Bot <!3509>
Part-of: <!3509>