drm/vc4: hdmi: Switch to drmm_kzalloc

Our internal structure that stores the DRM entities structure is allocated
through a device-managed kzalloc.

This means that this will eventually be freed whenever the device is
removed. In our case, the most likely source of removal is that the main
device is going to be unbound, and component_unbind_all() is being run.

However, it occurs while the DRM device is still registered, which will
create dangling pointers, eventually resulting in use-after-free.

Switch to a DRM-managed allocation to keep our structure until the DRM
driver doesn't need it anymore.

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