drm/etnaviv: Drop unused data member from the etnaviv_gem_object structure

Referencing the 'struct etnaviv_gpu *' by every etnaviv GEM BO is weird.
Drop it and drop yet another unused data field, namely 'access'.

Memory footprint of etnaviv GEM BOs reduced.

Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>