drm/i915: Use intel_de_rmw() for DDI clock routing

The DDI clock routing programming is riddled with shared
registers, forcing us to do a lot of RMW. Switch over to
intel_de_rmw() to make that a bit less obnoxious.

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/20210205214634.19341-9-ville.syrjala@linux.intel.com