drm/nouveau: fix reference count leak in nv50_disp_atomic_commit

nv50_disp_atomic_commit() calls calls pm_runtime_get_sync and in turn
increments the reference count. In case of failure, decrement the
ref count before returning the error.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>