Allow vblank interrupts to remain disabled across VT switch.

i915_driver_irq_postinstall was forcing vblank interrupts to pipe A when
called with vblank interrupts disabled. This caused vblank interrupts to be
accidentally re-enabled when VT switching the X server. Instead, start the
driver with vblank interrupts enabled on pipe A to support older X servers,
but then leave control over the state to the X server if it is able to do so.