drm/i915: Start moving the cdclk stuff into a distinct state structure

Introduce intel_cdclk state which for now will track the cdclk
frequency, the vco frequency and the reference frequency (not sure we
want the last one, but I put it there anyway). We'll also make the
.get_cdclk() function fill out this state structure rather than
just returning the current cdclk frequency.

One immediate benefit is that calling .get_cdclk() will no longer
clobber state stored under dev_priv unless ex[plicitly told to do
so. Previously it clobbered the vco and reference clocks stored
there on some platforms.

We'll expand the use of this structure to actually precomputing the
state and whatnot later.

v2: Constify intel_cdclk_state_compare()
v3: Document intel_cdclk_state_compare()
v4: Deal with i945gm_get_cdclk()

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/20170207183345.19763-1-ville.syrjala@linux.intel.com