drm/i915: Extract _cnl_ddi_{enable,disable}_clock()

All the DPCLKA_CFGCR handling follows a common pattern. Let's
extract that to a small helper that just takes a few parameters
each caller can customize.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210205214634.19341-12-ville.syrjala@linux.intel.com