drm/i915; Only increment the user-pin-count after successfully pinning the bo

drm/i915; Only increment the user-pin-count after successfully pinning the bo

commit 93be8788e648817d62fda33e2998eb6ca6ebf3a3 upstream.

As along the error path we do not correct the user pin-count for the
failure, we may end up with userspace believing that it has a pinned
object at offset 0 (when interrupted by a signal for example).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>