drm/etnaviv: avoid runtime PM usage in etnaviv_gpu_bind

Nothing in this callpath actually touches the GPU, so there is no reason
to get it out of suspend state here. Only if runtime PM isn't enabled at
all we must make sure to enable the clocks, so the GPU init routine can
access the GPU later on.

This also removes the need to guard against the state where the driver
isn't fully initialized yet in the runtime PM resume handler.

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