drm/msm/disp/dpu1: program 3d_merge only if block is attached

Update the 3d merge as active in the data path only if
the hw block is selected in the configuration.

Reported-by: Stephen Boyd <swboyd@chromium.org>
Fixes: 73bfb790 ("msm:disp:dpu1: setup display datapath for SC7180 target")
Signed-off-by: Kalyan Thota <kalyan_t@codeaurora.org>
Message-Id: <1617364493-13518-1-git-send-email-kalyan_t@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>