drm/vc4: perfmon: Add missing mutex_destroy

vc4_perfmon_open_file() will instantiate a mutex for that file instance,
but we never call mutex_destroy () in vc4_perfmon_close_file().

Let's add that missing call.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>