drm: Fix up kerneldoc for new drm_gem_dmabuf_export()

I hit send before completing a make htmldoc, and lo I forgot to fix up
the cut'n'paste.

Fixes: a4fce9cb ("drm/prime: Take a ref on the drm_dev when exporting...")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161005174056.29869-1-chris@chris-wilson.co.uk