drm/i915: Fix locking in DRRS debugfs

Grab the crtc mutex so that looking at the crtc state is
actually safe.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003113249.16213-5-ville.syrjala@linux.intel.com