drm/i915/tgl: Add Wa_1608008084

Wa_1608008084 is an additional WA that applies to writes on FF_MODE2
register. We can't read it back either from CPU or GPU. Since the other
bits should be 0, recommendation to handle Wa_1604555607 is to actually
just write the timer value.

Do a write only and don't try to read it, neither before or after
the WA is applied.

Fixes: ff690b21

 ("drm/i915/tgl: Implement Wa_1604555607")
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224191258.15668-1-lucas.demarchi@intel.com
(cherry picked from commit e94bda14

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>