Kernel: Simplify graphics initialization somewhat

We use a switch-case statements to ensure we try to find the best
suitable driver for a specific graphics card. In case we don't find
such, we use the default statement to initialize the graphics card as a
generic VGA adapter, if the adapter is VGA compatible.

If we couldn't initialize the driver, we don't touch this adapter
anymore.

Also, GraphicsDevice should not be tied to a PCI::Address member, as it
can be theortically be used with other buses (e.g. ISA cards).

Kernel/Graphics/VirtIOGPU/VirtIOGraphicsAdapter.cpp

@@ -17,7 +17,7 @@ NonnullRefPtr<VirtIOGraphicsAdapter> VirtIOGraphicsAdapter::initialize(PCI::Addr
 }
 
 VirtIOGraphicsAdapter::VirtIOGraphicsAdapter(PCI::Address base_address)
-    : GraphicsDevice(base_address)
+    : PCI::DeviceController(base_address)
 {
     m_gpu_device = adopt_ref(*new VirtIOGPU(base_address)).leak_ref();
 }