Reapply "drm/i915: Avoid stalling on pending flips for legacy cursor updates"

Reapply "drm/i915: Avoid stalling on pending flips for legacy cursor updates"

This reapplies commit acf4e84d6167317ff21be5c03e1ea76ea5783701.
With async unpin this should no longer break.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-20-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>