drm/i915/guc: Introduce unified HXG messages

New GuC firmware will unify format of MMIO and CTB H2G messages.
Introduce their definitions now to allow gradual transition of
our code to match new changes.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616001302.84233-2-matthew.brost@intel.com