drm/i915/gvt: Remove MMIO barrier in MMIO switch

After engine mmio switched, software still need write workload
submission registers. So we can remove the MMIO barriar in MMIO
switch.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>