drm/i915/psr: Fall back to max. synchronization latency if DPCD read fails

drm/i915/psr: Fall back to max. synchronization latency if DPCD read fails

Noticed that we assume the best case of 0 latency when the DPCD read
fails, reasonable pessimism is safer.

eDP spec does say that if latency is greater than 8, the panel
supplier needs to provide it. I didn't see anything specific in the VBT
for this, so let's go with 8 frames as a fallback.

Cc: Jose Roberto de Souza <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Reviewed-by: José Roberto de Souza <[email protected]>
Signed-off-by: Dhinakaran Pandiyan <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]