nb/intel/sandybridge/northbridge: Initialize uma_memory_base in all cases

Issue observed:
Coreboot stops at: "Not enough MTRRs available!"

Test system:
* Gigabyte GA-B75M-D3H
* Intel Pentium CPU G2130
* ATI Radeon HD4780

Problem description:
In case the IGD does not claim VGA decode, the code path taken results
in an integer overflow as uma_memory_base isn't initialized.
The MTRR assignment will fail, because of invalid memory regions.

Problem solution:
Properly initialize uma_memory_base to prevent possible integer overflow.

Final testing results:
The system boots again with IGD not claiming VGA decode.

Change-Id: I025be23b1defb6155469a3eee66569e49a695e7f
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: http://review.coreboot.org/11918
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/northbridge/intel/sandybridge/northbridge.c

@@ -194,6 +194,7 @@ static void pci_domain_set_resources(device_t dev)
 
 	printk(BIOS_DEBUG, "MEBASE 0x%llx\n", me_base);
 
+	uma_memory_base = tolud;
 	tomk = tolud >> 10;
 	if (me_base == tolud) {
 		/* ME is from MEBASE-TOM */