drm/i915: Avoid pageflipping freeze when we miss the flip prepare interrupt

When we miss the flip prepare interrupt, we never get into the
software state needed to restart userspace, resulting in a freeze of a
full-screen OpenGL application (such as a compositor).

Work around this by checking DSPxSURF/DSPxBASE to see if the page flip
has actually happened. If it has, do the work we would have done when
the flip prepare interrupt comes in.

Also, add debugfs information to tell us what's going on (based on the
patch from Chris Wilson attached to bugs.fdo bug #29798).

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>