drm/etnaviv: better track GPU state

Instead of only tracking if the FE is running, use a enum to better
describe the various states the GPU can be in. This allows some
additional validation to make sure that functions that expect a
certain GPU state are only called when the GPU is actually in that
state.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>