drm/i915: keep CONFIG_DRM_FBDEV_EMULATION=n function stubs together

Move the outcast intel_fbdev_output_poll_changed() stub for
CONFIG_DRM_FBDEV_EMULATION=n next to its friends.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475567628-5529-1-git-send-email-jani.nikula@intel.com