UMat allocation in CUDA targets causes unused CUDA devices (as OpenCL devices) to be initialized by these unexpected allocations in OpenCV 4. This PR stops these unwanted allocations from taking place if they are not required.

resolves #17852

Pull Request Readiness Checklist

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

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to 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

allow_multiple_commits=1

force_builders=Custom,Custom Win,Custom Mac
build_image:Custom=ubuntu-openvino-2020.4.0:16.04
build_image:Custom Win=openvino-2020.3.0
build_image:Custom Mac=openvino-2020.3.0

test_modules:Custom=dnn,python2,python3,java
test_modules:Custom Win=dnn,python2,python3,java
test_modules:Custom Mac=dnn,python2,python3,java

buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*