drm/etnaviv: add missing failure path to destroy suballoc

When something goes wrong in the GPU init after the cmdbuf suballocator
has been constructed, we fail to destroy it properly. This causes havok
later when the GPU is unbound due to a module unload or similar.

Fixes: e66774dd (drm/etnaviv: add cmdbuf suballocator)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Russell King <rmk+kernel@armlinux.org.uk>