drm/i915: Streamline VLV forcewake handling

It occured to me that when we're trying to wake up both render
and media wells on VLV, we might end up calling the low level
force_wake_get/put two times even though one call would be
enough. Make that happen by figuring out which wells really
need to be woken up based on the forcewake counts.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>