Claude review: drm/hibmc: Use gem-shmem with shadow-plane helpers for memory management

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

Patch Review

**Status: Good**

This is the core conversion. The approach closely follows ast and mgag200, which gives confidence it's well-proven.

**Core API change — exporting `drm_gem_shmem_create_with_handle()`**: This was previously static in `drm_gem_shmem_helper.c`. The export is needed because hibmc needs custom pitch alignment (128 bytes) in `dumb_create`, so it can't use the standard `drm_gem_shmem_dumb_create()`. Good documentation is added. The function is also declared in the header. Clean.

**VRAM management**: The old `drmm_vram_helper_init()` + TTM is replaced with a simple `devm_ioremap_wc()`:
```c
priv->vram = devm_ioremap_wc(dev->dev, vram_base, vram_size);
```
Write-combining is the correct mapping for VRAM that will be written by CPU memcpy.

**Shadow plane damage handling** in `atomic_update`:
```c
if (drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE) == 0) {
    drm_atomic_helper_damage_iter_init(&iter, old_state, new_state);
    drm_atomic_for_each_plane_damage(&iter, &damage) {
        struct iosys_map dst[DRM_FORMAT_MAX_PLANES] = {
            IOSYS_MAP_INIT_VADDR_IOMEM(priv->vram + gpu_addr),
        };
        iosys_map_incr(&dst[0],
                       drm_fb_clip_offset(fb->pitches[0], fb->format, &damage));
        drm_fb_memcpy(dst, fb->pitches, shadow_plane_state->data, fb, &damage);
    }
    drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
}
```

This follows the ast pattern exactly. The `drm_gem_fb_begin_cpu_access()` / `end_cpu_access()` bracket properly synchronizes dma-buf shared buffers. If `begin_cpu_access` fails, the function silently skips the copy but still programs the hardware registers — this is acceptable since `atomic_update` can't return errors.

**`gpu_addr` is hardcoded to 0**: Since there's no VRAM allocator anymore, all framebuffers are copied to VRAM offset 0. This means no double-buffering in VRAM, so page flips will show tearing during the copy. This is the same tradeoff made by ast/mgag200 and is acceptable for a server BMC display card. Damage clipping (`drm_plane_enable_fb_damage_clips`) minimizes the copy window.

**Mode validation**:
```c
const struct drm_format_info *info =
    drm_get_format_info(dev, DRM_FORMAT_XRGB8888, DRM_FORMAT_MOD_LINEAR);
```
Using XRGB8888 (the largest format at 4 bpp) gives a conservative check ensuring any supported format will fit. The overflow-safe division `pitch > max_fb_size / mode->vdisplay` is the correct approach.

**Kconfig changes**: Drops `DRM_VRAM_HELPER`, `DRM_TTM`, `DRM_TTM_HELPER`, adds `DRM_GEM_SHMEM_HELPER`. Clean.

No issues found.

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