timedelta64[ns] axis tick formatting problem

#### ALL software version info

hvplot 0.6.0

#### Description of expected behavior and the observed behavior

It looks like tick formatting for time axes is not being applied for the first plot in a layout, only in subsequent ones:

```python
import pandas as pd
import holoviews as hv
import hvplot.pandas
import numpy as np

hv.extension('bokeh')

time_end = 10
sampling_time = 0.2

timepoints = np.arange(0, time_end, sampling_time)

df = pd.DataFrame(dict(
time=pd.to_timedelta(timepoints, 's'),
value=np.sin(timepoints),
value_2=timepoints,
))

(
df.hvplot(x='time', y='value')
+ df.hvplot(x='time', y='value_2')
)
```