drm/i915/dc3co: Do the full calculation of DC3CO exit only once

This will calculaet the DC3CO exit delay only once per full modeset.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200122182617.18597-1-jose.souza@intel.com