drm/i915/cx0: Toggle powerdown states for C10 on HDMI

drm/i915/cx0: Toggle powerdown states for C10 on HDMI

A new step has been added to Bspec with respect to the C10 PHY, which
instructs the driver to toggle powerdown value for boths PHY lanes to P0
and then P2 when driving an HDMI connector. This update in the Bspec
reflects the changes required by Wa_14026084006, so document it.

Note that, unlike other display workarounds, this one is actually tied
to the C10 PHY and not to a specific display IP.  As such, let's just
document it in intel_cx0_phy.c instead of adding it to
intel_display_wa.c.

Bspec: 64568, 74489
Reviewed-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Link: https://patch.msgid.link/20251216-wa_14026084006-c10-hdmi-toggle-powerdown-v1-2-08677b03e2f1@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>