drm/xe: clear the serviced bits on INTR_IDENTITY_REG

The spec for this register, like many other interrupt related ones,
asks software to write back '1' to clear the serviced bits. Let's
respect the spec.

v2:
- Update commit message
- Add missing CC

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
CC: Daniele Spurio Ceraolo <daniele.ceraolospurio@intel.com>
CC: Lucas De Marchi <lucas.demarchi@intel.com>
CC: Rodrigo Vivi <rodrigo.vivi@intel.com>
CC: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>