drm/i915: Polish intel_shared_dpll_swap_state()

Use swap() instead of hand rolling it in intel_shared_dpll_swap_state(),
and pass in the intel_atomic_state instead of drm_atomic_state. Makes
the code less convoluted.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190701160550.24205-3-ville.syrjala@linux.intel.com


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