System information (version)

• OpenCV => 4.5
• Operating System / Platform => Ubuntu 18.04
• Compiler => gcc 7.5

Detailed description

While doing some performance comparisons between cv::Mat and cv::UMat (OpenCL), I noticed that OpenCL was taking a lot longer (8x) when performing color conversions from YUV to BGR or RGB.

I compared my benchmarking code to the perf tests, and I was not populating the optional dstCn parameter in my calls to cv::cvtColor(). When I did pass that parameter, my performance greatly improved!

I think I tracked it down to the function dstChannels() in color.hpp

I don't fully understand why passing 0 as the value for dcn to the OpenCL kernel results in such poor performance, but seems like a simple fix.

Steps to reproduce

```.cpp
cv::UMat yuv = cv::UMat::zeros(1920, 1080, CV_8UC3);
cv::UMat rgb;
cv::cvtColor(yuv, rgb, cv::COLOR_YUV2RGB); /// This is slow :-(
cv::cvtColor(yuv, rgb, cv::COLOR_YUV2RGB, 3); /// This is fast!
```

I can whip up a complete test case/perf test if that's helpful.

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc