Fix issues with VA_INCLUDE_HEADERS when building with CUDA support

[dmatveev]

Issue description

• For some reason, WITH_ is used instead of HAVE_ when checking for VA libs in G-API CMakeLists
• WITH_VA is TRUE by default on x86 (see opencv/CMakeLists.txt:414)
• When configuring samples, the test for VA haven't been made (but WITH_VA is still true and it is ok)
• Removed redundant call to OpenCVFindVA.cmake as it is done anyway (see opencv/CMakeLists.txt:743 and opencv/cmake/OpenCVFindLibsVideo.cmake)
• Removed FATAL ERROR case from G-API's CMakeLists.txt if VA isn't available. Have no idea why it is there

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 another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• 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

[SergeyIvanov87]

LGTM

But I would recommend to add warning because vpl on Linux Is useless without va_api

[alalek]

opencv_test_gapi

Test target should be guarded, see code below.

[dmatveev]

Good point, updated. Thanks

[dmatveev]

Also tested the build with VA being available:

root@7973346b3aad:~/opencv/build# ldd bin/example_gapi_onevpl_infer_single_roi | grep va
        libva.so.2 => /lib/x86_64-linux-gnu/libva.so.2 (0x00007fc3af609000)
        libva-drm.so.2 => /lib/x86_64-linux-gnu/libva-drm.so.2 (0x00007fc3af604000)

At least it builds now

[mshabunin]

For some reason, WITH_ is used instead of HAVE_ when checking for VA libs in G-API CMakeLists
Removed redundant call to OpenCVFindVA.cmake as it is done anyway

Perhaps it was designed like this for standalone G-API build. Do we still support this configuration?

[dmatveev]

@mshabunin yes we do, but VPL & VAAPI were never a part of it. Good question if it is still tested on the CI though..