drm/i915/mtl: add GSC CS interrupt support

The GSC CS re-uses the same interrupt bits that the GSC used in older
platforms. This means that we can now have an engine interrupt coming
out of OTHER_CLASS, so we need to handle that appropriately.

v2: clean up the if statement for the engine irq (Tvrtko)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221102171047.2787951-4-daniele.ceraolospurio@intel.com