drm/i915/guc: Unify notify() functions

The Gen11+ and the legacy function differ in the register and value
written to interrupt the GuC. However, while on older gen the value
matches a bit on the register, on Gen11+ the value is a SW defined
payload that is sent to the FW. Since the FW behaves the same no matter
what value we pass to it, we can just write the same thing on all gens
and get rid of the function pointer by saving the register offset.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217012316.13271-6-daniele.ceraolospurio@intel.com