drm/etnaviv: fix some off by one bugs

The ->nr_signal is the supposed to be the number of elements in the
->signal array.  There was one place where it was 5 but it was supposed
to be 4.  That looks like a copy and paste bug.  There were also two
checks that were off by one.

Fixes: 9e2c2e27 ("drm/etnaviv: add infrastructure to query perf counter")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>