drm/i915: Confine page flips to BCS on Valleyview

Once again we find that Valleyview is ever so subtlety different from
the rest of its gen7 brethen. In this case, Valleyview has no support
for pageflipping from the RCS ring.

Fixes a regression from

commit ffe74d75
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Aug 26 20:58:12 2013 +0100

    drm/i915: Use RCS flips on Ivybridge+

Reported-by: "Lee, Chon Ming" <chon.ming.lee@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68968


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>