drm/i915/guc: Correctly handle GuC interrupts on Gen11

Starting Gen11 GuC shares interrupt registers with SG unit
instead of PM. But for now we don't care about SG interrupts.

v2: (Chris)
v3: rebased (Michal)
v4: more bspec pages, use macros, update commit msg (Michal Wi)

Bspec: 19820, 19840, 19841, 20176

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527183613.17076-13-michal.wajdeczko@intel.com