drm/i915/hdcp: Move intel_hdcp_gsc_message def away from header file

Move intel_hdcp_gsc_message definition into intel_hdcp_gsc.c
so that intel_hdcp_gsc_message can be redefined for xe as needed.

--v2
-Correct commit message to reflect what patch is actually doing [Arun]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240306024741.1858039-2-suraj.kandpal@intel.com