nb/intel/gm45/gma: Centralize call to gm45_get_lvds_edid_str()

There is only a single place where we need the LVDS EDID string. Let's
call gm45_get_lvds_edid_str() right there. This simplifies the API and
helps to follow the execution flow.

The function is moved to avoid a forward declaration.

Change-Id: I86f3a88e6b661bcf60319edbe301e70304924727
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75378
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
diff --git a/src/mainboard/lenovo/x200/blc.c b/src/mainboard/lenovo/x200/blc.c
index 655be56..99def22d 100644
--- a/src/mainboard/lenovo/x200/blc.c
+++ b/src/mainboard/lenovo/x200/blc.c
@@ -38,7 +38,7 @@
 
 const char *mainboard_vbt_filename(void)
 {
-	u16 pwm_freq = get_blc_pwm_freq_value(gm45_get_lvds_edid_str());
+	u16 pwm_freq = get_blc_pwm_freq_value();
 
 	if (pwm_freq == 0) {
 		printk(BIOS_DEBUG,