drm/i915/psr: Share the computation of idle frames

Both activate functions and the dc3co disable function were doing the
same thing, so better move to a function and share.
Also while at it adding a WARN_ON to catch invalid values.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200113214603.52158-1-jose.souza@intel.com