Allow accessing the parent structure later on. Drop const for allowing
future modification as well.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3318b09aaccbbe141e233ca510d581b421259a2a.1575974743.git.jani.nikula@intel.com