etnaviv: move dummy BOs to screen

etnaviv: move dummy BOs to screen

The dummy texture descriptor and the dummy render target relocs are not ever
changed by a context operation, so we can save some space by moving them to
the screen and potentially share them and the BOs backing them between
multiple contexts.

Also don't hold two pointers to the same BO, one in the reloc and one raw,
but always just use the reloc one.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17448>