drm/msm/dpu: don't overwrite CTL_MERGE_3D_ACTIVE register

drm/msm/dpu: don't overwrite CTL_MERGE_3D_ACTIVE register

In case of complex pipelines (e.g. the forthcoming quad-pipe) the DPU
might use more that one MERGE_3D block for a single output.  Follow the
pattern and extend the CTL_MERGE_3D_ACTIVE active register instead of
simply writing new value there. Currently at most one MERGE_3D block is
being used, so this has no impact on existing targets.

Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/641580/
Link: https://lore.kernel.org/r/20250307-dpu-active-ctl-v3-1-5d20655f10ca@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>