drm/etnaviv: move GPU active handling to bo pin/unpin

The active count is used to check if the BO is idle, where idle is defined
as not active on the GPU and all VM mappings and reference counts dropped
to the initial state. As the idling of the mappings and references now only
happens in the submit cleanup, the active state handling must be moved to
the same location in order to keep the userspace semantics.

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