x2apic: Remove a panic condition in enabling x2APIC

x2apic: Remove a panic condition in enabling x2APIC

Currently Xen triggers a panic if user disables VT-d by command line
while not disable x2APIC. This requires users to specify both
"iommu=0" and "x2apic=0" to disable VT-d if the platform supports
x2APIC. It's not user friendly. This patch removes the panic
condition. That's to say, don't require user to specify "x2apic=0"
when specify "iommu=0". As long as VT-d is not enabled (disabled in
BIOS or in command line), x2APIC won't be enabled naturally (x2APIC
depends on VT-d Interrupt remapping).

Signed-off-by: Weidong Han <weidong.han@intel.com>