drm/i915: Add psr_safest_params

This parameter is meant to be used when PSR issues are found as some
issues in the past was due wrong values set in VBT so this would be
a quick and easy way to ask users or for us to check if the issue is
due VBT values.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200520212756.354623-1-jose.souza@intel.com