drm/i915: Add missing else to the if ladder in missing else

The if ladder in gen8_de_pipe_fault_mask() was missing one
else, add it. Doesn't actually matter since each if branch
just returns directly. But the code is less confusing when
you always do things the same way.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217070047.953-2-ville.syrjala@linux.intel.com