drm/i915/hdmi: make hdcp2_msg_data const

It's static const data, make it so.

Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820134019.13229-5-jani.nikula@intel.com