drm/i915: Lock gmbus/aux mutexes while changing cdclk

gmbus/aux may be clocked by cdclk, thus we should make sure no
transfers are ongoing while the cdclk frequency is being changed.
We do that by simply grabbing all the gmbus/aux mutexes. No one
else should be holding any more than one of those at a time so
the lock ordering here shouldn't matter.

v2: Use mutex_lock_nest_lock() (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200302174442.5803-1-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>