[IA64] Don't alloc empty frame in ia64_switch_mode_phys

I think ia64_switch_mode_phys and ia64_switch_mode_virt
does not need to alloc an empty frame.
An empty frame is required by loadrs but flushrs
does not need that.

Signed-off-by: Zou Nan hai <nanhai.zou@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>