drm/i195: Extract cnl_ddi_{enable,disable}_clock()

Extract the DDI clock routing for CNL into the new vfuncs.

v2: s/dev_priv/i915/ (Lucas)

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/20210205214634.19341-6-ville.syrjala@linux.intel.com