Fix this sparse warning:

  drivers/gpu/drm/i915/intel_fb.c:417:5: warning: symbol 'intelfb_panic' was not declared. Should it be static?
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NDave Airlie <airlied@linux.ie>