[mlir] Expose MLIR_CUDA_CONVERSIONS_ENABLED in mlir-config.h. (#83004)

That macro was not defined in some cases and thus yielded warnings if
compiled with `-Wundef`. In particular, they were not defined in the
BUILD files, so the GPU targets were broken when built with Bazel. This
commit exposes mentioned CMake variable through mlir-config.h and uses
the macro that is introduced with the same name. This replaces the macro
MLIR_CUDA_CONVERSIONS_ENABLED, which the CMake files previously defined
manually.