drm/vc4: kms: Don't duplicate pending commit

Our HVS global state, when duplicated, will also copy the pointer to the
drm_crtc_commit (and increase the reference count) for each FIFO if the
pointer is not NULL.

However, our atomic_setup function will overwrite that pointer without
putting the reference back leading to a memory leak.

Since the commit is only relevant during the atomic commit process, it
doesn't make sense to duplicate the reference to the commit anyway.
Let's remove it.

Fixes: 9ec03d7f

 ("drm/vc4: kms: Wait on previous FIFO users before a commit")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>