Claude review: drm/rockchip: dw_dp: Implement out-of-band HPD handling

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

Patch Review

This is the largest patch in the series and significantly reworks `dw_dp-rockchip.c`.

**Issue — hardcoded IO addresses for controller identification:**
```c
+static const struct rockchip_dw_dp_plat_data rk3588_dp_plat_data = {
+	.ctrl_ids = {0xfde50000, 0xfde60000},
```
The controller ID is determined by matching the platform resource start address against hardcoded values. This works but is fragile — it should ideally come from the DT (e.g., a `reg` index or an explicit property). This pattern appears in some other Rockchip drivers but is generally considered suboptimal.

**Issue — `dp_dp_rockchip_get_vo_grf()` is a workaround:**
```c
+	/*
+	 * Limit this workaround to RK3576 and RK3588, new platforms should
+	 * add a VO GRF phandle in the DisplayPort DT node.
+	 */
```
The code navigates from the DP node → PHY phandle → PHY's `rockchip,vo-grf` to find the VO GRF. The comment acknowledges this is a workaround. This is acceptable for existing DTs but the DT binding should be updated to add a direct `rockchip,vo-grf` property for future platforms.

**Minor — `#include <linux/hw_bitfield.h>`:** This is a relatively new header. The `FIELD_PREP_WM16_CONST` usage is correct for Rockchip's write-masked GRF registers.

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