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

Extract the DDI clock routing clode for skl/derivatives
into the new encoder 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/20210205214634.19341-5-ville.syrjala@linux.intel.com