drm/i915/debugfs: Increment return value of gt.next_seqno

The i915_next_seqno read value is to be the next seqno used by the
kernel. However, in the conversion to atomics ops for gt.next_seqno, in
commit 28176ef4cfa5 ("drm/i915: Reserve space in the global seqno during
request allocation"), this was changed from a post-increment to a
pre-increment. This increment was missed from the value reported by
debugfs, so in effect it was reporting the current seqno (last
assigned), not the next seqno.

Fixes: 28176ef4cfa5 ("drm/i915: Reserve space in the global seqno during request allocation")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81209
Signed-off-by: Chris Wilson <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Joonas Lahtinen <[e-mail]>
(cherry picked from commit 9607ae79710afb453173b90d5bf564788a6e09b1)
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/i915/i915_debugfs.c

···
 {
 	struct drm_i915_private *dev_priv = data;
 
-	*val = atomic_read(&dev_priv->gt.global_timeline.next_seqno);
+	*val = 1 + atomic_read(&dev_priv->gt.global_timeline.next_seqno);
 	return 0;
 }