drm/i915/icl: Correctly clear lost ctx-switch interrupts across reset for Gen11

Interrupt handling in Gen11 is quite different from previous platforms.

v2: Rebased (Michel)
v3: Rebased with wiggle
v4: Rebased, remove TODO warning correctly (Daniele)
v5: Rebased, made gen11_gtiir const while at it (Michel)
v6: Rebased
v7: Adapt to the style currently in upstream

Suggested-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.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/1524605995-22324-1-git-send-email-oscar.mateo@intel.com