drm/i915: Save the old CDCLK atomic state

drm/i915: Save the old CDCLK atomic state

BugLink: https://launchpad.net/bugs/1826868
The old state will be needed by an upcoming patch to determine if the
commit increases or decreases CDCLK, so move the old state to the atomic
state (while keeping the new one in dev_priv). cdclk.logical and
cdclk.actual in the atomic state isn't used atm anywhere after the
atomic check phase, so this should be safe.

v2:
- Use swap() instead of opencoding it. (Ville)

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190320135439.12201-2-imre.deak@intel.com
(backported from commit 48d9f87ddd2108663fd866b254e05d422243cc56)
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Acked-by: AceLan Kao <acelan.kao@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>