drm/i915: Decouple the stuck pageflip on modeset

drm/i915: Decouple the stuck pageflip on modeset

If we successfully confuse the hardware, and cause it to drop a queued
pageflip, we wait for 60s and issue a warning before continuing on with
the modeset. However, this leaves the pending pageflip still stuck
indefinitely. Pretend to userspace that it does complete, and let us
start afresh following the modeset.

v2: Rebase after refactor
v3: Rebase, rebase.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=82612
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>