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: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>