x86/platform/uv: Use x2apic enabled bit as set by BIOS to indicate APIC mode

BIOS now sets the x2apic enabled bit (and the ACPI table) for extended
APIC modes. Use that bit to indicate if extended mode is set.

 [ bp: Fixup subject prefix, merge subsequent fix
   https://lkml.kernel.org/r/20210415220626.223955-1-mike.travis@hpe.com

 ]

Signed-off-by: Mike Travis <mike.travis@hpe.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210408160047.1703-1-mike.travis@hpe.com