This addresses WaPruneModeWithIncorrectHsyncOffset.

Bugzilla: http://bugs.freedesktop.org/show_bug.cgi?id=50236Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>