drm/i915: Adjust eDP's logical vco in a reliable place.

On intel_dp_compute_config() we were calculating the needed vco
for eDP on gen9 and we stashing it in
intel_atomic_state.cdclk.logical.vco

However few moments later on intel_modeset_checks() we fully
replace entire intel_atomic_state.cdclk.logical with
dev_priv->cdclk.logical fully overwriting the logical desired
vco for eDP on gen9.

So, with wrong VCO value we end up with wrong desired cdclk, but
also it will raise a lot of WARNs: On gen9, when we read
CDCLK_CTL to verify if we configured properly the desired
frequency the CD Frequency Select bits [27:26] == 10b can mean
337.5 or 308.57 MHz depending on the VCO. So if we have wrong
VCO value stashed we will believe the frequency selection didn't
stick and start to raise WARNs of cdclk mismatch.

[   42.857519] [drm:intel_dump_cdclk_state [i915]] Changing CDCLK to 308571 kHz, VCO 8640000 kHz, ref 24000 kHz, bypass 24000 kHz, voltage level 0
[   42.897269] cdclk state doesn't match!
[   42.901052] WARNING: CPU: 5 PID: 1116 at drivers/gpu/drm/i915/intel_cdclk.c:2084 intel_set_cdclk+0x5d/0x110 [i915]
[   42.938004] RIP: 0010:intel_set_cdclk+0x5d/0x110 [i915]
[   43.155253] WARNING: CPU: 5 PID: 1116 at drivers/gpu/drm/i915/intel_cdclk.c:2084 intel_set_cdclk+0x5d/0x110 [i915]
[   43.170277] [drm:intel_dump_cdclk_state [i915]] [hw state] 337500 kHz, VCO 8100000 kHz, ref 24000 kHz, bypass 24000 kHz, voltage level 0
[   43.182566] [drm:intel_dump_cdclk_state [i915]] [sw state] 308571 kHz, VCO 8640000 kHz, ref 24000 kHz, bypass 24000 kHz, voltage level 0

v2: Move the entire eDP's vco logical adjustment to inside
    the skl_modeset_calc_cdclk as suggested by Ville.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: bb0f4aab ("drm/i915: Track full cdclk state for the logical and actual cdclk frequencies")
Cc: <stable@vger.kernel.org> # v4.12+
Link: https://patchwork.freedesktop.org/patch/msgid/20180502175255.5344-1-rodrigo.vivi@intel.com