xen: remove stray preempt_disable() from PV AP startup code

In cpu_bringup() there is a call of preempt_disable() without a paired
preempt_enable(). This is not needed as interrupts are off initially.
Additionally this will result in early boot messages like:

BUG: scheduling while atomic: swapper/1/0/0x00000002

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210825113158.11716-1-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>