Implement support for the VBLANK IOCTL. Note that Tegra is somewhat
special in this case because it doesn't use the generic IRQ support
provided by the DRM core (DRIVER_HAVE_IRQ) but rather registers one
interrupt handler for each display controller.

While at it, clean up the way that interrupts are enabled to ensure
that the VBLANK interrupt only gets enabled when required.
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>