drm/etnaviv: simplify unbind checks

Remember if the GPU has been sucessfully initialized. Only in that case
do we need to clean up various structures in the unbind path. If the
GPU hasn't been sucessfully initialized all the cleanups should happen
in the failure paths of the init function.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>