drm/msm/dpu: Don't always set merge_3d pending flush

Don't set the merge_3d pending flush bits if the mode_3d is
BLEND_3D_NONE.

Always flushing merge_3d can cause timeout issues when there are
multiple commits with concurrent writeback enabled.

This is because the video phys enc waits for the hw_ctl flush register
to be completely cleared [1] in its wait_for_commit_done(), but the WB
encoder always sets the merge_3d pending flush during each commit
regardless of if the merge_3d is actually active.

This means that the hw_ctl flush register will never be 0 when there are
multiple CWB commits and the video phys enc will hit vblank timeout
errors after the first CWB commit.

[1] commit fe9df3f5 ("drm/msm/dpu: add real wait_for_commit_done()")

Fixes: 3e79527a ("drm/msm/dpu: enable merge_3d support on sm8150/sm8250")
Fixes: d7d0e73f ("drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback")
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/619092/
Link: https://lore.kernel.org/r/20241009-mode3d-fix-v1-1-c0258354fadc@quicinc.com


Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>