drm/i915: Add the missing HDMI gamut metadata packet stuff

We have definitions and low level code for everything except the gamut
metadata HDMI packet. Add the missing bits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225174106.2163-2-ville.syrjala@linux.intel.com