drm: fix error found in some cases after the patch d1af5cd86997

The patch d1af5cd86997 ("drm: get rid of DRM_DEBUG_* log
calls in drm core, files drm_a*.c") fails when the drm_device
cannot be found in the parameter plane_state->crtc.
Fix it using plane_state->plane.

Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: 6e22dc35 ("drm: get rid of DRM_DEBUG_* log calls in drm core, files drm_a*.c")
Signed-off-by: Claudio Suarez <cssk@net-c.es>
[danvet: fix Fixes: line]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/YaiXEARd8z2C463h@gineta.localdomain