drm/i915/vgpu: Neuter forcewakes for VGPU more thoroughly

If we avoid initializing forcewake domains when running as
a guest, and also use gen2 mmio accessors in that case, we
can avoid the timer traffic and any looping through the
forcewake code which is currently just so it can end up in
the no-op forcewake implementation.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Weinan Li <weinan.z.li@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Terrence Xu <terrence.xu@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170310095747.12258-1-tvrtko.ursulin@linux.intel.com
[tursulin: commit spelling fix]