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

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 <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161124094752.19129-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
(cherry picked from commit 9607ae79710afb453173b90d5bf564788a6e09b1)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>