Fix unfilled kdeplot cmap regression

[mwaskom]

Fixes #3058

geyser = sns.load_dataset("geyser")
sns.kdeplot(geyser, x="waiting", y="duration", cmap="viridis")

[codecov]

Codecov Report

Merging #3065 (15015bc) into master (c412ddf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3065   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          72       72           
  Lines       23537    23540    +3     
=======================================
+ Hits        23156    23159    +3     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/distributions.py	96.35% <100.00%> (-0.02%)	⬇️
seaborn/utils.py	95.44% <100.00%> (+0.01%)	⬆️
tests/test_distributions.py	99.79% <100.00%> (+<0.01%)	⬆️