drm/i915: Do the fbc vs. primary plane enable/disable in the right order

Disable fbc before disabling the primary plane, and enable fbc after
the primary plane has been enabled again.

Also use intel_disable_fbc() to disable FBC to avoid the pointless
overhead of intel_update_fbc(), and especially avoid having to clean
up and set up the stolen mem compressed buffer again.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>