drm/i915/gvt: Add runtime pm around fences

Manipulating the fence_list requires the runtime wakelock, as does
writing to the fence registers. Acquire a wakelock for the former, and
assert that the device is awake for the latter.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>