drm/vc4: dpi: Add action to disable the clock

The DPI controller has two clocks called core and pixel, the core clock
being enabled at bind time.

Adding a device-managed action will make the error path easier, so let's
create one to disable it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>