i965/gen9: Only allow Y-Tiled MCS buffers

i965/gen9: Only allow Y-Tiled MCS buffers

For GEN9, much of the logic to use X-Tiled buffers has been stripped out. It is
still supported in some places, but it's never desirable. Unfortunately we don't
yet have the ability to have Y-Tiled scanout (see:
http://patchwork.freedesktop.org/patch/46984/),

NOTE: This patch shouldn't actually do anything since SKL doesn't yet use fast
clears (they are disabled because they are causing regressions). THerefore, the
only case we can get to this function on SKL is by way of
intel_update_winsys_renderbuffer_miptree.

v2: Update commit message to be more clear that the NOTE is for SKL only.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chad Versace <chad.versace@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>