soc/intel/common: refactor locate_vbt

All callers of locate_vbt just care about the file content and
immediately map the rdev for its content.
Instead of repeating this in all call sites, move that code to
locate_vbt.

Change-Id: I5b518e6c959437bd8f393269db7955358a786719
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/21896
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/drivers/intel/fsp2_0/graphics.c

@@ -99,8 +99,7 @@ uintptr_t fsp_load_vbt(void)
 	struct region_device rdev;
 	void *vbt_data = NULL;
 
-	if (locate_vbt(&rdev) != CB_ERR)
-		vbt_data = rdev_mmap_full(&rdev);
+	vbt_data = locate_vbt(&rdev);
 
 	if (vbt_data == NULL)
 		printk(BIOS_NOTICE, "Could not locate a VBT file in CBFS\n");