Provide support for cmake 2.8 and align cmake requirements across sources

Some Linux distributions, especially server distributions, still has older cmake versions than 3.5 by default. Cent OS 7.4 is an example:
```
# cmake --version
cmake version 2.8.12.2
```
Yocto is another example:
```
# cmake --version
cmake version 3.3.1

CMake suite maintained and supported by Kitware (kitware.com/cmake).
```
Please, provide compatibility with older versions of cmake starting from 2.8 to extend Linux distributions coverage.

Also, please, align cmake version requirements across sources. As it stands now:
```
# fgrep -rsn cmake_minimum_required .
./Tools/MediaDriverTools/GenKrnBin/CMakeLists.txt:21:cmake_minimum_required(VERSION 2.6)
./Tools/MediaDriverTools/KernelBinToSource/CMakeLists.txt:21:cmake_minimum_required (VERSION 2.8)
./Tools/MediaDriverTools/KrnToHex_IGA/CMakeLists.txt:21:cmake_minimum_required(VERSION 2.6)
./cmrtlib/CMakeLists.txt:22:cmake_minimum_required(VERSION 2.8)
./cmrtlib/linux/CMakeLists.txt:21:cmake_minimum_required(VERSION 2.8)
./media_driver/CMakeLists.txt:22:cmake_minimum_required(VERSION 3.5)
./CMakeLists.txt:21:cmake_minimum_required(VERSION 3.5)
```

Mind that I have submitted similar request against gmmlib: https://github.com/intel/gmmlib/issues/1