drm/i915: Fix cdclk bypass freq readout for tgl/bxt/glk

On tgl/bxt/glk the cdclk bypass frequency depends on the PLL
reference clock. So let's read out the ref clock before we
try to compute the bypass clock.

Cc: Matt Roper <matthew.d.roper@intel.com>
Fixes: 71dc367e ("drm/i915: Consolidate bxt/cnl/icl cdclk readout")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911133129.27466-1-ville.syrjala@linux.intel.com