drm/i915/gvt: Add runtime_pm_get/put into gvt_switch_mmio

drm/i915/gvt: Add runtime_pm_get/put into gvt_switch_mmio

If user continuously create vgpu, boot guest, shoutdown guest and destroy
vgpu from remote, the following calltrace exists in dmesg sometimes:
[ 6412.954721] RPM wakelock ref not held during HW access
[ 6412.954795] WARNING: CPU: 7 PID: 11941 at
linux/drivers/gpu/drm/i915/intel_drv.h:1800
intel_uncore_forcewake_get.part.7+0x96/0xa0 [i915]
[ 6412.954915] Call Trace:
[ 6412.954951] intel_uncore_forcewake_get+0x18/0x20 [i915]
[ 6412.954989] intel_gvt_switch_mmio+0x8e/0x770 [i915]
[ 6412.954996] ? __slab_free+0x14d/0x2c0
[ 6412.955001] ? __slab_free+0x14d/0x2c0
[ 6412.955006] ? __slab_free+0x14d/0x2c0
[ 6412.955041] intel_vgpu_stop_schedule+0x92/0xd0 [i915]
[ 6412.955073] intel_gvt_deactivate_vgpu+0x48/0x60 [i915]
[ 6412.955078] __intel_vgpu_release+0x55/0x260 [kvmgt]

when this happens, gvt_switch_mmio is called at vgpu destroy, host i915 is
idle and doesn't hold RPM wakelock, igd is in powersave mode, but
gvt_switch_mmio require igd power on to access register, so
intel_runtime_pm_get should be added to make sure igd power on before
gvt_switch_mmio.

v2: Move runtime_pm_get/put into gvt_switch_mmio.(Zhenyu)

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>