drm/msm/disp/dpu: add merge3d support for sc7280

drm/msm/disp/dpu: add merge3d support for sc7280

On SC7280 targets, display modes with a width greater than the
max_mixer_width (2400) are rejected during mode validation when
merge3d is disabled. This limitation exists because, without a
3D merge block, two layer mixers cannot be combined(non-DSC interface),
preventing large layers from being split across mixers. As a result,
higher resolution modes cannot be supported.

Enable merge3d support on SC7280 to allow combining streams from
two layer mixers into a single non-DSC interface. This capability
removes the width restriction and enables buffer sizes beyond the
2400-pixel limit.

Fixes: 591e34a091d1 ("drm/msm/disp/dpu1: add support for display for SC7280 target")
Signed-off-by: Mahadevan P <mahadevan.p@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/696713/
Link: https://lore.kernel.org/r/20260101-4k-v2-1-712ae3c1f816@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>