drm/i915: protect FBC functions with FBC checks

Now all the functions called by other files check whether FBC has been
initialized. This allows us to drop the checks on the static
functions.

v2:
 - s/HAS_FBC/dev_priv->display.enable_fbc/ everywhere but the init
   function (Chris).

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>