drm/etnaviv: disable tx clock gating for GC7000 rev6203

The i.MX8MN SoC errata sheet mentions ERR050226: "GPU: Texture L2 Cache
idle signal may incorrectly clock gate the texture engine in GPU".

The workaround is to disable the corresponding clock gatings.

While on it move the clock gating check for rev6202 into the same check
to bundle them.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>