i915: Avoid mis-counting vblank interrupts when they're only enabled for pipe A.

It looks like 'after a while', I915REG_INT_IDENTITY_R for some reason always has
VSYNC_PIPEB_FLAG set in the interrupt handler, even though pipe B is disabled.
So we only increase dev->vbl_received if the corresponding bit is also set in
dev->vblank_pipe.
(cherry picked from 881ba569 commit)