drm/i915/guc: Handle default action received over CT

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