drm/i915: swap() the entire cdclk state

To make life less confusing let's swap() the entire cdclk state
rather than swapping some parts, copying other parts, and leaving
the rest just as is.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-11-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>