drm/i915/psr: Move psr specific dpcd init into own function

This patch is preparing adding panel replay specific dpcd init.

Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231108072303.3414118-3-animesh.manna@intel.com