drm/msm: fix NULL-deref on snapshot tear down

In case of early initialisation errors and on platforms that do not use
the DPU controller, the deinitilisation code can be called with the kms
pointer set to NULL.

Fixes: 98659487

 ("drm/msm: add support to take dpu snapshot")
Cc: stable@vger.kernel.org      # 5.14
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/525099/
Link: https://lore.kernel.org/r/20230306100722.28485-4-johan+linaro@kernel.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>