etnaviv: fix refcnt initialization in etna_screen

etnaviv: fix refcnt initialization in etna_screen

Despite being a member of the etna_screen struct, 'refcnt' is used by
the winsys-specific logic to track the reference count of the object
managed in a hash table. When the count reaches zero, the pipe screen
is removed from the table and destroyed.

Fix the logic by initializing the refcnt to 1 when screen created.
This initialization is done in etna_screen_create(), to follow the
same logic as in freedreno and virgl.

Fixes: c9e8b49b885 ("etnaviv: gallium driver for Vivante GPUs")
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
(cherry picked from commit 5d8514de14bd27170293bb373e06f5ff43c708ad)