drm/i915/tgl: Add note about Wa_1409142259

Different issues with the same fix, so justing adding
Wa_1409142259, Wa_1409252684, Wa_1409217633, Wa_1409207793,
Wa_1409178076 and 1408979724 to the comment so other devs can check if
this Was were implemetend with a simple grep.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227220101.321671-8-jose.souza@intel.com