drm/vc4: hvs: Remove planes currently allocated before taking down

When the HVS driver is unbound, a lot of memory allocations in the LBM and
DLIST RAM are still assigned to planes that are still allocated.

Thus, we hit a warning when calling drm_mm_takedown() since the memory pool
is not completely free of allocations.

Let's free all the currently live entries before calling drm_mm_takedown().

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