ihdgd: implement GMBUS write operations

Replaced GMBUS WRITE TODO stub with real implementation.
GMBUS write works like read but writes data to register 3
instead of reading from it. Handles sub-4-byte chunks by
reconstructing a u32 before writing (the GMBUS data register
expects 32-bit writes).

Cross-referenced with Linux 7.1 i915 display/intel_gmbus.c
gmbus_xfer_write() which writes bytes to the GMBUS data
register with HW_RDY polling. Enables display configuration
writes (brightness, color settings, panel parameters) on
Intel GPU platforms via the GMBUS I2C/SMBus interface.

drivers/graphics/ihdgd/src/device/gmbus.rs

@@ -109,8 +109,29 @@ impl<'a> Transactional for GmbusPinPair<'a> {
                     }
                 }
                 Operation::Write(buf) => {
-                    log::warn!("TODO: GMBUS WRITE");
-                    return Err(());
+                    for chunk in buf.chunks(4) {
+                        {
+                            // Wait for hardware ready before writing
+                            let timeout = Timeout::from_millis(10);
+                            while !self.regs[2].readf(GMBUS2_HW_RDY) {
+                                timeout.run().map_err(|()| {
+                                    log::debug!(
+                                        "timeout on GMBUS write 0x{:08x}",
+                                        self.regs[2].read()
+                                    );
+                                    ()
+                                })?;
+                            }
+                        }
+
+                        // Write data to GMBUS data register (index 3).
+                        // Must reconstruct a u32 because the register
+                        // expects 32-bit writes even for sub-4-byte chunks.
+                        let mut word = 0u32;
+                        let len = chunk.len().min(4);
+                        word.to_le_bytes()[..len].copy_from_slice(chunk);
+                        self.regs[3].write(word);
+                    }
                 }
             }
         }