drm/nouveau/fifo: small cleanup in nvkm_chan_cctx_get()

"&chan->cgrp->mutex" and "&cgrp->mutex" are the same thing.  Use
"&cgrp->mutex" consistently.  It looks nicer and it silences a
Smatch static checker warning.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/aBHaCM66pXaP84ei@stanley.mountain


Signed-off-by: Danilo Krummrich <dakr@kernel.org>