drm/i915/gvt: Fix pipe A enable as default for vgpu

observed igt drv_module_reload test case failure on 4.15.0
rc2 kernel with panic due to no active pipe available.

the gpu will reset during unload/load and make pipe config reg
lost which can cause kernel panic issue happen.

this patch is to move pipe enabling to emulate_mointor_status_chagne
to handle vgpu reset case as well.

Fixes: 7e605902

 ("drm/i915/gvt: enabled pipe A default on creating vgpu")
Signed-off-by: Xiaolin Zhang <xiaolin.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>