When running on platform with CTB based GuC communication enabled,
GuC to Host event data will be delivered as CT request message.
However, content of the data[1] of this CT message follows format
of the scratch register used in MMIO based communication, so some
code reuse is still possible.

v2:  filter disabled messages (Daniele)
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com> #1
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Acked-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327214124.70680-1-michal.wajdeczko@intel.com