drm/i915: Keep MCHBAR always enabled

As we need more and more controls within MCHBAR for memory config and
power management, this trys to keep MCHBAR enabled from driver load and
only tear down in driver unload.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>