drm/etnaviv: fix crash in GPU suspend when init failed due to buffer placement

drm/etnaviv: fix crash in GPU suspend when init failed due to buffer placement

When the suballocator was unable to provide a suitable buffer for the MMUv1
linear window, we roll back the GPU initialization. As the GPU is runtime
resumed at that point we need to clear the kernel cmdbuf suballoc entry to
properly skip any attempt to manipulate the cmdbuf when the GPU gets shut
down in the runtime suspend later on.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>