drm/i915: Disable FBC on Ironlake to save 1W

Frame buffer compression is broken on Ironlake due to buggy hardware.
Currently it is disabled through chicken bits, but it still consumes
over 1W more than if we simply never attempt to enable the FBC code
paths.

Signed-off-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org