drm/i915: Remove duplicated bxt/cnl/icl .modeset_calc_cdclk() funcs

Reuse the same .modeset_calc_cdclk() function for all bxt+.

The only difference in between the cnl/icl and the bxt variants
is the call to cnl_compute_min_voltage_level(). We can do that call
just fine on older platforms since they leave min_voltage_level[]
zeroed. Let's rename the function to bxt_compute_min_voltage_level()
just so it stays consistent with the rest of the naming scheme.

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/20190911133129.27466-4-ville.syrjala@linux.intel.com