drm/i915: Sanitize prepare_pipes after valleyview_modeset_global_pipes()

valleyview_modeset_global_pipes() may add pipes that are getting fully
disabled to prepare_pipes bitmask. The rest of the code doesn't expect
this, so clear out any such pipes from the prepare_pipes bitmask.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>