drm/i915: Convert intel_vgt_(de)balloon to uncore

More removal of implicit dev_priv from using old mmio accessors.

Furthermore these calls really operate on ggtt so it logically makes sense
if they take it as parameter.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-1-tvrtko.ursulin@linux.intel.com