drm/i915/gvt: optimize for vGPU mmio switch

Now mmio switch between vGPUs need to switch to host first then to expected
vGPU, it waste one time mmio save/restore. r/w mmio usually is
time-consuming, and there are so many mocs registers need to save/restore
during vGPU switch. Combine the switch_to_host and switch_to_vgpu can
reduce 1 time mmio save/restore, it will reduce the CPU utilization and
performance while there is multi VMs with heavy work load.

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>