etnaviv: fix vertex buffer state emission for single stream GPUs

GPUs with a single supported vertex stream must use the single state
address to program the stream.

Fixes: 3d09bb39 (etnaviv: GC7000: State changes for HALTI3..5)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Jonathan Marek <jonathan@marek.ca>
(cherry picked from commit ce23bc92)