drm/i915: FBC off for ironlake and older, otherwise on by default

Make the default FBC behaviour chipset specific, allowing us to turn
it on by default for Ironlake and older where it has been seen to
cause trouble with screen updates.

Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Francis Moreau <francis.moro@gmail.com>