drm/i915: Track full cdclk state for the logical and actual cdclk frequencies

The current dev_cdclk vs. cdclk vs. atomic_cdclk_freq is quite a mess.
So here I'm introducing the "actual" and "logical" naming for our
cdclk state. "actual" is what we'll bash into the hardware and "logical"
is what everyone should use for state computaion/checking and whatnot.
We'll track both using the intel_cdclk_state as both will need other
differing parameters than just the actual cdclk frequency.

While doing that we can at the same time unify the appearance of the
.modeset_calc_cdclk() implementations a little bit.

v2: Commit dev_priv->cdclk.actual since that already has the
    new state by the time .modeset_commit_cdclk() is called.
v3: s/locical/logical/ and improve the docs a bit

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170120182205.8141-9-ville.syrjala@linux.intel.com