drm/i915/vlv: save/restore the power context base reg

Some BIOSes (e.g. the one on the Minnowboard) don't save/restore this
reg.  If it's unlocked, we can just restore the previous value, and if
it's locked (in case the BIOS re-programmed it for us) the write will be
ignored and we'll still have "did it move" sanity check in the PM code to
warn us if something is still amiss.

References: https://bugs.freedesktop.org/show_bug.cgi?id=89611


Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Darren Hart <dvhart@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>