Claude review: drm/vmwgfx: Check vrefresh in drm_mode_setcrtc.

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

Patch Review

**Status: Looks good, but missing Fixes/Cc stable tags**

The patch adds `drm_mode_config_funcs.mode_valid` to validate modes set via the legacy `drm_mode_setcrtc` path, which the existing per-connector `mode_valid` doesn't cover. The vrefresh check (`if (!drm_mode_vrefresh(mode)) return MODE_BAD`) is also added to the STDU connector's `vmw_stdu_connector_mode_valid`.

Good decisions in this patch:
- Moving `GPU_TILE_SIZE` from `vmwgfx_stdu.c` to `vmwgfx_kms.h` for reuse.
- Casting the `ALIGN` result to `u64` to avoid 32-bit overflow in `required_mem` calculation:
  ```c
  required_mem = (u64)ALIGN(mode->hdisplay, GPU_TILE_SIZE) *
                 ALIGN(mode->vdisplay, GPU_TILE_SIZE) *
                 assumed_cpp;
  ```

**Missing tags**: This patch fixes `kms_invalid_mode@overflow-vrefresh` but has no `Fixes:` tag or `Cc: stable@` annotation. If this is a regression from a specific commit, a Fixes tag would help with backporting. Even without a clear single culprit, a Cc: stable would be appropriate if this matters for older kernels.

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