Claude review: drm/msm/dpu: support plane splitting in quad-pipe case

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Review: Core logic change, mostly correct but with some concerns.**

This extends `dpu_plane_split()` to iterate over multiple mixer stages. Key logic:
- `num_stages = (num_lm + 1) / 2` for virtual planes (1 stage for 1-2 LMs, 2 stages for 3-4 LMs)
- Each stage covers a horizontal band of the display
- For each stage, the plane's dst_rect is clipped against the mixer rectangle
- Within each stage, a wide-plane split may further divide into 2 pipes

Concerns:

1. **`dpu_crtc_get_num_lm()` is called before topology is finalized in some paths.** The function reads `cstate->num_mixers`. In patch 2, `dpu_plane_split()` is called from `dpu_plane_assign_resources()` / `dpu_plane_virtual_assign_resources()`, which happens during CRTC check after `dpu_crtc_assign_resources()` has run. So `num_mixers` should be set by then. But this ordering dependency is subtle and fragile.

2. **`dpu_plane_atomic_check_sspp()` now loops over all `PIPES_PER_PLANE` (4) entries.** This is correct but changes from checking exactly pipe[0] and pipe[1] to checking any non-zero-width pipe_cfg. For the non-virtual case with `num_stages=1`, only pipe_cfg[0] and pipe_cfg[1] are populated, and pipe_cfg[2]/pipe_cfg[3] should be zero-initialized from `dpu_plane_state` allocation. This should be safe.

3. **The `memset(pipe_cfg, 0, 2 * sizeof(struct dpu_sw_pipe_cfg))` at line 943** clears both pipe_cfg and r_pipe_cfg when the plane doesn't intersect a mixer rect. This relies on `pipe_cfg` and `r_pipe_cfg` being contiguous in the array, which they are (`pipe_cfg[cfg_idx]` and `pipe_cfg[cfg_idx + 1]`). Correct but uses a fragile assumption about memory layout.

4. **The mixer_rect calculation** `stage_id * mode->hdisplay / num_stages` uses integer division. For odd hdisplay values with num_stages=2, the split might not be perfectly symmetric (e.g., 1081/2 = 540 vs 541). This should match the hardware configuration but is worth verifying.

Has Reviewed-by from Dmitry Baryshkov and Jessica Zhang. **Minor concerns noted above.**

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