[Fix] Fix potential interger overflow in imequalize.

[mzr1996]

Motivation

imequalize in mmcv have a potential integer overflow bug and get result like the following image:

While in Pillow, it's right.

Modification

Conver all numbers in lut greater than 255 to 255.

BC-breaking (Optional)

No

[codecov]

Codecov Report

Merging #1198 (a5a28d8) into master (6659c38) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1198   +/-   ##
=======================================
  Coverage   68.00%   68.00%           
=======================================
  Files         160      160           
  Lines       10442    10443    +1     
  Branches     1902     1902           
=======================================
+ Hits         7101     7102    +1     
  Misses       2968     2968           
  Partials      373      373           

Flag	Coverage Δ
unittests	68.00% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/image/photometric.py	99.27% <100.00%> (+<0.01%)	⬆️

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