drm/i915/bxt: Update max level of vswing

Broxton supports 3 voltage swing levels on all DP ports.
Max level of pre-emphasis will be taken care with the existing code.

v2: Patch rebased

v3: (imre)
- keep existing behavior for other platforms
- clarify commit message

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com> (v2)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>