drm/i915/dp: move TPS3 logic to where it's used

There is no need to have a separate flag for tps3 as the information is
only used at one location. Move the logic there to make it easier to
follow.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>