drm/i915/dg1: add support for the master unit interrupt

DG1 has master unit interrupt register which is used to indicate the
correct source of interrupt.

v2: fix coding style on register definition

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Daniele Spurio Ceraolo <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200713182321.12390-4-lucas.demarchi@intel.com