i915: Protect vblank IRQ reg access with spinlock

This uses the same spinlock as the user_irq code as it shares the same
register, ensuring that interrupt registers are updated atomically.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>