drm/nouveau/ibus/gk20a: use udelay() in interrupt context

gk20a_ibus_init_ibus_ring() can be called from gk20a_ibus_intr(), in
non-interruptible context. Replace use of usleep_range() with udelay().

Reported-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>