drm/etnaviv: add security handling mode enum

With the introduction of GPU security we have 3 different modes of
GPU operation:
- GPU core doesn't have security features -> no handling required
- the security related states are handled by the kernel driver
- the security related states are handled by a TrustZone application

Add a enum to differentiate between the different operation modes.

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