Kernel: Move all Graphics-related code into Devices/GPU directory

Like the HID, Audio and Storage subsystem, the Graphics subsystem (which
handles GPUs technically) exposes unix device files (typically in /dev).
To ensure consistency across the repository, move all related files to a
new directory under Kernel/Devices called "GPU".

Also remove the redundant "GPU" word from the VirtIO driver directory,
and the word "Graphics" from GraphicsManagement.{h,cpp} filenames.

Kernel/Devices/GPU/GenericGraphicsAdapter.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <Kernel/Bus/PCI/Definitions.h>
+#include <Kernel/Devices/BlockDevice.h>
+#include <Kernel/Library/LockWeakable.h>
+#include <Kernel/Memory/PhysicalAddress.h>
+
+namespace Kernel {
+class GenericGraphicsAdapter
+    : public AtomicRefCounted<GenericGraphicsAdapter>
+    , public LockWeakable<GenericGraphicsAdapter> {
+public:
+    virtual ~GenericGraphicsAdapter() = default;
+
+protected:
+    GenericGraphicsAdapter() = default;
+};
+
+}