drm/i915: Convert cdclk to global state

Let's convert cdclk_state to be a proper global state. That allows
us to use the regular atomic old vs. new state accessor, hopefully
making the code less confusing.

We do have to deal with a few more error cases in case the cdclk
state duplication fails. But so be it.

v2: Fix new plane min_cdclk vs. old crtc min_cdclk check

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121140353.25997-1-ville.syrjala@linux.intel.com


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