etnaviv: add etna_pipe_wait_ns(..)

We need to pass through a timeout parameter to implement
pipe->fence_finish() properly. The new fxn accepts a timeout
in nanoseconds. Simplify etna_pipe_wait(..) by using
etna_pipe_wait_ns(..).

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>