Use enum class for MemoryPool in GraphicsAllocation
This change will ensure that GA is constructed in the proper way
- Rename namespace for isSystemMemoryPool method
- Add method getMemoryPoolString for logging actual pool which is in used
- Remove wrong pattern in GraphicsAllocation constructor
Related-To: NEO-6523
Signed-off-by: Krzysztof Gibala <krzysztof.gibala@intel.com>