drm/i915: Enhance cdclk sanitization

When reading out the BIOS-programmed cdclk state, let's make sure that
the cdclk value is on the valid list for the platform, ensure that the
VCO matches the cdclk, and ensure that the CD2X divider was set
properly.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910154252.30503-8-matthew.d.roper@intel.com