nb/intel/haswell/gma.c: Use `config_of` in `gma_setup_panel`

This is to reduce differences between Haswell and Broadwell.

Change-Id: I8d6a8ee02e24bee22f0a7b69098ea8430095ba90
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46689
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/northbridge/intel/haswell/gma.c

@@ -287,7 +287,7 @@ static void init_display_planes(void)
 
 static void gma_setup_panel(struct device *dev)
 {
-	struct northbridge_intel_haswell_config *conf = dev->chip_info;
+	struct northbridge_intel_haswell_config *conf = config_of(dev);
 	u32 reg32;
 
 	printk(BIOS_DEBUG, "GT Power Management Init (post VBIOS)\n");