Make sure no VBlank interrupts happen when LCD is turned off

[RaoulLuque]

It can happen that the VBlank flag is set when the LCD is still on whilst the VBlank enable flag is not set (IME is set). Then the LCD is turned and off later whilst the LCD is still off, the VBlank enable flag is set. In that case, there should (?) be no VBlank interrupt since the LCD/PPU is off