drm/vc4: Switch to drmm_mutex_init

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