drm/i915: Extract intel_modeset_calc_cdclk()

Exfiltrate the cdclk code from intel_modeset_checks() into
intel_modeset_calc_cdclk().

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913193157.9556-4-ville.syrjala@linux.intel.com