drm/i915: Only export the generic intel_disable_fbc() interface

As the enable/disable routines will be gain additional complexity in
future patches, it is necessary that all callers do not bypass the
generic interface by calling into the chipset routines directly. to do
this we make the chipset routines static, so there is no choice.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>