drm: Fixup kernel doc for driver->gem_create_object

Silences

./include/drm/drm_drv.h:295: warning: Incorrect use of kernel-doc format

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161125123427.15188-1-chris@chris-wilson.co.uk