Fix histogram viewer with dask arrays

[astrofrog]

Fixes #2148

@keflavich - can you confirm this works for you?

[keflavich]

Yep, that fixed it.

[codecov]

Codecov Report

Merging #2149 into master will decrease coverage by 0.02%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master    #2149      +/-   ##
==========================================
- Coverage   87.87%   87.85%   -0.03%     
==========================================
  Files         246      246              
  Lines       22596    22609      +13     
==========================================
+ Hits        19857    19863       +6     
- Misses       2739     2746       +7     

Impacted Files	Coverage Δ
glue/core/data.py	90.22% <31.25%> (-1.16%)	⬇️
glue/viewers/matplotlib/qt/widget.py	81.81% <0.00%> (+3.40%)	⬆️

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