matlab: Use OpenCV_SOURCE_DIR instead of CMAKE_SOURCE_DIR to find the modules root

[Qubiz]

This fixes #3312

Previously the OpenCV modules was found using CMAKE_SOURCE_DIR which always points to the top level CMake directory.

Using OpenCV_SOURCE_DIR allows for a more robust way of getting the path to the OpenCV modules root and ensures that the HDR_PARSER_PATH is correctly set.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• [x] I agree to contribute to the project under Apache 2 License.
• [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• [x] The PR is proposed to the proper branch
• [x] There is a reference to the original bug report and related work
• [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Patch to opencv_extra has the same branch name.
• [ ] The feature is well documented and sample code can be built with the project CMake