etnaviv: GC7000: Factor out state based texture functionality

etnaviv: GC7000: Factor out state based texture functionality

Prepare for two texture handling paths, the descriptor-based
path will be added in a future commit. These are structured
so that the texture implementation handles its own state
emission.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>