cpu/x86/lapic: Fix X2APIC_ONLY regression

Some ancient CPUs may have had LAPIC disabled at power-up, so
semantically enable_lapic() should always come before attempting
to access the register banks.

With X2APIC_ONLY option it is necessary to ensure enable_lapic()
is called prior to any other lapic register space accesses,
since the XAPIC mode MMIO accessors are optimised away build-time
and CPU's do not yet initialise for X2APIC mode at reset.

Change-Id: I96eaa5c43108c802375e184e0c68b5091ca0198f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76195
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/cpu/x86/mtrr/mtrr.c

@@ -806,6 +806,8 @@ static void _x86_setup_mtrrs(unsigned int above4gb)
 {
 	int address_size;
 
+	enable_lapic();
+
 	x86_setup_fixed_mtrrs();
 	address_size = cpu_phys_address_size();
 	printk(BIOS_DEBUG, "apic_id 0x%x setup mtrr for CPU physical address size: %d bits\n",