ocl: avoid unnecessary loading/initializing OpenCL subsystem

If there are no OpenCL/UMat methods calls from application.

OpenCL subsystem is initialized:
- haveOpenCL() is called from application
- useOpenCL() is called from application
- access to OpenCL allocator: UMat is created (empty UMat is ignored) or UMat <-> Mat conversions are called

Don't call OpenCL functions if OPENCV_OPENCL_RUNTIME=disabled
(independent from OpenCL linkage type)

modules/core/src/umatrix.cpp

@@ -141,7 +141,7 @@ void UMatData::unlock()
 MatAllocator* UMat::getStdAllocator()
 {
 #ifdef HAVE_OPENCL
-    if( ocl::haveOpenCL() && ocl::useOpenCL() )
+    if (ocl::useOpenCL())
         return ocl::getOpenCLAllocator();
 #endif
     return Mat::getDefaultAllocator();