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 <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511195145.3829-5-dhinakaran.pandiyan@intel.com