drm/i915: Make i9xx_load_ycbcr_conversion_matrix() static

Make i9xx_load_ycbcr_conversion_matrix() static to appease sparse:
intel_color.c:110:6: warning: symbol 'i9xx_load_ycbcr_conversion_matrix' was not declared. Should it be static?

Cc: Shashank Sharma <shashank.sharma@intel.com>
Fixes: 25edf915 ("drm/i915: prepare csc unit for YCBCR420 output")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901143123.7590-1-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_color.c

@@ -107,7 +107,7 @@ static void ctm_mult_by_limited(uint64_t *result, int64_t *input)
 	}
 }
 
-void i9xx_load_ycbcr_conversion_matrix(struct intel_crtc *intel_crtc)
+static void i9xx_load_ycbcr_conversion_matrix(struct intel_crtc *intel_crtc)
 {
 	int pipe = intel_crtc->pipe;
 	struct drm_i915_private *dev_priv = to_i915(intel_crtc->base.dev);