drm/vc4: dpi: Return an error if we can't enable our clock

If we fail to enable the DPI clock, we just ignore the error and moves
forward. Let's return an error instead.

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