drm/i915: Sprinkle a few missing locks around shared DDI clock registers

The current code attempts to protect the RMWs into global
clock routing registers with a mutex, but forgets to do so
in a few places. Let's remedy that.

Note that at the moment we serialize all modesets onto single
wq, so this shouldn't actually matter. But maybe one day we
wish to attempt parallel modesets again...

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/20210205214634.19341-10-ville.syrjala@linux.intel.com