drm/i915/tgl: Add note about Wa_1607063988

This issue workaround in Wa_1607063988 has the same fix as
Wa_1607138336, so just adding a note in the code.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227220101.321671-6-jose.souza@intel.com