Kernel: Make device generate their own names

Besides removing the monolithic DevFSDeviceInode::determine_name()
method, being able to determine a device's name inside the /dev
hierarchy outside of DevFS has its uses.

Kernel/Devices/MBVGADevice.cpp

@@ -114,4 +114,9 @@ int MBVGADevice::ioctl(FileDescription&, unsigned request, FlatPtr arg)
     };
 }
 
+String MBVGADevice::device_name() const
+{
+    return String::formatted("fb{}", minor());
+}
+
 }