drm/i915: noop forcewake get/put when vgpu activated

For a virtualized GPU, the host maintains the forcewake state on the real
device. As we don't control forcewake ourselves, we can simply set
force_wake_get() and force_wake_put() to be no-ops. By setting the vfuncs,
we adjust both the manual control of forcewake and around the mmio
accessors (making our vgpu specific mmio routines redundant and to be
removed in the next patch).

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1485408013-12780-1-git-send-email-weinan.z.li@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>