drm/i915: Move PFI credit reprogramming into vlv/chv_set_cdclk()

Move the vlv_program_pfi_credits() into vlv_set_cdclk() and
chv_set_cdclk() so that we can neuter vlv_modeset_commit_cdclk().

v2: Do the PFI programming after cdclk readout since it currently
    depends on the readout to fill dev_priv->cdclk.hw

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/20170126195719.309-1-ville.syrjala@linux.intel.com