drm/i915: add vlv power wells

Based on an early draft from Jesse.

Add support for powering on/off the dynamic power wells on VLV by
registering its dynamic power wells with the power domain framework.

I tested this so far only with VGA output and I have to double check the
HW state save/restore part as well as the need to force some clocks on
during power toggling. For all this Jesse has already some code posted
to intel-gfx, so that should be used as a basis.

Signed-off-by: Imre Deak <imre.deak@intel.com>