ARM: vfp: ensure that thread flushing works if preempted

ARM: vfp: ensure that thread flushing works if preempted

Prevent a preemption event causing the initialized VFP state being
overwritten by ensuring that the VFP hardware access is disabled
prior to starting initialization.  We can then do this in safety
while still allowing preemption to occur.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>