etnaviv: Consolidate buffer references from framebuffers

We were leaking surfaces because the references taken in
etna_set_framebuffer_state weren't being released on context destroy.

Instead of just directly releasing those references in
etna_context_destroy, use the util_copy_framebuffer_state helper.

Take the chance to remove the duplicated buffer references in
compiled_framebuffer_state to avoid confusion.

The leak can be reproduced with a client that continuously creates and
destroys contexts.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reported-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
(cherry picked from commit bf1dfcc3)
[Emil: resolve trivial conflict - dummy_rt does not exist in branch]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/gallium/drivers/etnaviv/etnaviv_context.c