Claude review: drm: bridge: cadence: Update mhdp8546 mailbox access functions

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

Patch Review

**Critical: `cdns_mhdp_reg_write()` behavioral change.** The old mhdp8546 `cdns_mhdp_reg_write()` used:
```c
cdns_mhdp_mailbox_send(mhdp, MB_MODULE_ID_DP_TX,
                       DPTX_WRITE_REGISTER, sizeof(msg), msg);
```
with a 16-bit address in a 6-byte message. The new helper uses:
```c
cdns_mhdp_mailbox_send(base, MB_MODULE_ID_GENERAL,
                       GENERAL_REGISTER_WRITE, sizeof(msg), msg);
```
with a 32-bit address in an 8-byte message. This changes the firmware command path from DP_TX to GENERAL module. While both may ultimately write to the same register space, the firmware could interpret them differently. **This needs explicit confirmation that the GENERAL_REGISTER_WRITE command is functionally equivalent for all the registers the mhdp8546 driver writes to.** This could cause a silent regression on existing mhdp8546 platforms.

**Similarly**, `cdns_mhdp_reg_read()` changed from a local function that directly calls send/recv_header/recv_data to using the new `cdns_mhdp_mailbox_send_recv()` helper. The old code used `MB_MODULE_ID_GENERAL` already, so this is fine.

**Good**: The `cdns_mhdp_base` initialization in probe is clean:
```c
mhdp->base.dev = mhdp->dev;
mhdp->base.regs = mhdp->regs;
mhdp->base.sapb_regs = mhdp->sapb_regs;
```

**Alignment nit:** Several `cdns_mhdp_mailbox_send_recv()` calls have inconsistent indentation of continuation arguments, e.g.:
```c
ret = cdns_mhdp_mailbox_send_recv(&mhdp->base, MB_MODULE_ID_GENERAL,
			  GENERAL_MAIN_CONTROL,
```
The second line should align under `&mhdp->base`.

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Generated by Claude Code Patch Reviewer