drm/i915/guc: Make event handler a virtual function

drm/i915/guc: Make event handler a virtual function

On platforms with CTB based GuC communications, we will handle
GuC events in a different way. Let's make event handler a virtual
function to allow easy switch between those variants.

Credits-to: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326194829.58836-6-michal.wajdeczko@intel.com