drm/etnaviv: hold GPU lock while inserting END command

Inserting the END command when suspending the GPU is changing the
command buffer state, which requires the GPU to be held.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>