[intel] Get vblank pipe from irq_mask_reg instead of hardware enable reg

[intel] Get vblank pipe from irq_mask_reg instead of hardware enable reg

With the interrupt enable/disable using only the mask register, it was wrong
to use the enable register to detect which pipes had vblank detection
turned on. Also, as we keep a local copy of the mask register around, and
MSI machines smack the hardware during the interrupt handler, it is more
efficient and more correct to use the local copy.