drm/i915: Add gen8_de_pipe_fault_mask()

Reduce the clutter a bit by introducing gen8_de_pipe_fault_mask().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626180344.26314-2-ville.syrjala@linux.intel.com