drm/i915: Handle PendingFlip on gen3 robustly

We appear to allow too many pending pageflips as evidenced by an
apparent pin-leak. So borrow the pageflip completion logic from i8xx for
handling PendingFlip in a robust manner.

v2: Address Jesse's reminders about the nuances of gen3 IRQ handling.

References: https://bugzilla.kernel.org/show_bug.cgi?id=41882


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>