drm/i915: Fix a couple of "should it be static?" sparse warnings

A genuine 'static' omission and 2 other warnings triggered by not
including the header where those functions where defined.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>