drm/msm/dpu: drop stale comment from struct dpu_mdp_cfg doc

drm/msm/dpu: drop stale comment from struct dpu_mdp_cfg doc

The field ubwc_static was removed from struct dpu_mdp_cfg some time ago.
Drop the corresponding kerneldoc now.

Fixes: 544d8b96150d ("drm/msm/dpu: update UBWC config for sm8150 and sm8250")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/519554/
Link: https://lore.kernel.org/r/20230121135457.2788199-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>