drm/i915/dp: use the drm dp helper for determining sink tps3 support

No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: s/intel_dp_tps/drm_dp_tps/.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>