drm: Add PSR version 3 macro

eDP 1.4a specification defines PSR version 3, it PSR2 with the
addition of Y-coordinate support when doing selective update.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180317013828.24182-1-jose.souza@intel.com