drm/managed: Fix off-by-one in warning

I'm thinking this is the warning that fired in the 0day report, but I
can't double-check yet since 0day didn't upload its source tree
anywhere I can check. And all the drivers I can easily test don't use
drm_dev_alloc anymore ...

Also if I'm correct supreme amounts of bad luck because usually kslap
(for bigger structures) gives us something quite a bit bigger than
what we asked for.

[0day uploaded tree, guess is correct]

Reported-by: kernel test robot <lkp@intel.com>
Fixes: c6603c74 ("drm: add managed resources tied to drm_device")
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Neil Armstrong <narmstrong@baylibre.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200328162358.18500-1-daniel.vetter@ffwll.ch