drm/i915:: Disable FBC on SandyBridge

Enabling FBC is causing the BLT ring to run between 10-100x slower than
normal and frequently lockup. The interim solution is disable FBC once
more until we know why.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>