drm/i915: Split adl-s/rkl from icl_ddi_combo_{enable,disable}_clock()

Since .{enable,disable}_clock() are already vfuncs it's a bit silly to
have if-ladders inside them. Just provide specialized version for adl-s
and rkl so we don't need any of that.

v2: s/dev_priv/i915/ (Lucas)
    Fix typos in platform names (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/20210205214634.19341-13-ville.syrjala@linux.intel.com