drm/i915: un-EXPORT and make 'intelfb_panic' static

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: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>