drm/i915: Disable power management for i915 driver in VM

With Intel GVT-g, GPU power management is controlled by
host driver, so there is no need to provide virtualized
GPU PM support. In the future it might be useful to gather
VM input for freq boost, but now let's disable it simply.

v2:
take Chris' comments:
        - do not special case this to gen6+

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>