drm/msm: Check for NULL before calling prepare_commit()

Add a NULL check before calling prepare_commit() in
msm_atomic_commit_tail()

Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/523604/
Link: https://lore.kernel.org/r/20230221184256.1436-3-quic_jesszhan@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>