drm/i915/bxt: Move DDI PHY enabling/disabling to the power well code

So far we depended on the HW to dynamically power down unused PHYs and
so we enabled them manually once during driver loading/resuming. There
are indications however that we can achieve better power savings by
manual powering toggling. So make the PHY enabling/disabling to happen
on-demand whenever we need either the corresponding AUX or port
functionality. CHV does this already by enabling the PHY along the
corresponding PHY common lane power wells there, do the same on BXT by
adding virtual power wells for the same purpose.

Also sanity check the common lane power down ack signal from the PHY. Do
this only when the PHY is enabled, since it's not clear at what point
the HW power/clock gates things.

While at it rename broxton_ prefix to bxt_ in related function names to
better align with the SKL code.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>