drm/i915/gen9: Assume CDCLK PLL is off if it's not locked

If the CDCLK PLL isn't locked or incorrectly configured we can just
assume that it's off resulting in fully re-initializing both CDCLK PLL
and CDCLK dividers. This way the CDCLK PLL sanitization added in the
following patch can be done on BXT the same way as it's done on SKL.

v2: (Ville)
- Remove the remaining PLL specific checks from skl_sanitize_cdclk() and
  depend instead on the corresponding check in skl_dpll0_update().
- Use vco == 0 instead of the corresponding boolean check in
  skl_sanitize_cdclk().

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464093513-16258-1-git-send-email-imre.deak@intel.com