Changing the mode will destoy the GBM surface for the output. As a result all
corresponding BOs are deleted regardless of the drm_fb refcount.
While a commit is pending, the last_state may contain a reference to such a BO.
So delay the mode switch until the commit is finished and the reference is
release.
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>