mutex_init is supposed to be balanced by a call to mutex_destroy that we
were never doing in the vc4 driver.

Since a DRM-managed mutex_init variant has been introduced, let's just
switch to it.
Acked-by: NThomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: NMaxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-66-maxime@cerno.tech