Heatmap xticks customisation doesn't work

#### ALL software version info
hvplot==0.7.0
panel==0.10.3
Python 3.8

#### Description of expected behavior and the observed behavior
xticks (default=None): int or list
Ticks along x specified as an integer, list of
ticks positions, or list of tuples of the tick positions and labels

Tick positions work as the description says, but labels do not. This issue is present with all three options described above. If default it does work, in all other cases the plot will always display the tick position as labels.
I've tried this in Jupyter Classic notebook and in PyCharm too. The symptoms are the same.

#### Complete, minimal, self-contained example code that reproduces the issue
The code below is from the examples page from the official website. I've just added the .opts part and using panel to display it.

```
import hvplot.pandas
from bokeh.sampledata.unemployment1948 import data
import panel as pn

data = data.set_index('Year').drop('Annual', axis=1).transpose()
x = data.hvplot.heatmap(
x='columns',
y='index',
title='US Unemployment 1948—2016',
cmap=["#75968f", "#a5bab7", "#c9d9d3", "#e2e2e2", "#dfccce", "#ddb7b1", "#cc7878", "#933b41", "#550b1d"],
xaxis='top',
rot=70,
width=800, height=300).opts(
toolbar=None,
fontsize={'title': 10, 'xticks': 12, 'yticks': 5}
)
x.opts(xticks=[(10, 'foo'), (20, 'bar'), (30, 'lol')])
pn.panel(x).show()
```

#### Stack traceback and/or browser JavaScript console output

#### Screenshots or screencasts of the bug in action