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: NThierry Reding <treding@nvidia.com>
Signed-off-by: NAlexandre Courbot <acourbot@nvidia.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/ibus/gk20a.c

@@ -29,7 +29,7 @@ gk20a_ibus_init_ibus_ring(struct nvkm_subdev *ibus)
 	nvkm_mask(device, 0x137250, 0x3f, 0);
 
 	nvkm_mask(device, 0x000200, 0x20, 0);
-	usleep_range(20, 30);
+	udelay(20);
 	nvkm_mask(device, 0x000200, 0x20, 0x20);
 
 	nvkm_wr32(device, 0x12004c, 0x4);