drm/i915: Make i9xx_load_ycbcr_conversion_matrix() static

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: 25edf91501b8 ("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.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>