drm/i915/bxt, glk: Give a proper name to the power well struct phy field

Follow-up patches will add new fields to the i915_power_well struct that
are specific to the hsw_power_well_ops helpers. Prepare for this by
changing the generic 'data' field to a union of platform specific
structs.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1499352040-8819-8-git-send-email-imre.deak@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>