drm/i915/bdw: Broadwell also has the "power down well"

Just like Haswell, but with the small twist that the panel fitter for pipe A is
now also in the always-on power well.

v2: Use the new HAS_POWER_WELL macro.

v3: Rebase on top of intel_using_power_well patches.

v4: This time actually update the PFIT check correctly so that the
pipe A pfit is in the always-on domain.

v5: Rebase on top of the VGA power domain addition.

v6: Rebase on top of the new power domain infrastructure. Also pimp the commit
message a bit while at it.

v7: Use IS_BROADWELL instead of IS_GEN8 (Ville).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>