Fixes subplots=True for geo=True and with by-statement

[hoxbro]

The second part (2/2) of the fix for https://discourse.holoviz.org/t/small-multiples-aka-facetted-plots/3760/2

This will make it possible to run (after https://github.com/holoviz/holoviews/pull/5325 is merged):

import geopandas as gpd
import hvplot.pandas

countries = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres'))
countries.hvplot(geo=True, by="continent", subplots=True)

Before fix (with https://github.com/holoviz/holoviews/pull/5325):

After fix: