drm/i915/dp: program VSwing and Preemphasis control settings on VLV v2

Program few Tx buffer Swing control settings through DPIO.

v2: fix up codingstyle (Daniel)
    call from set_signal_levels (Ville, Daniel)
    use proper port numbers (Jesse)

Signed-off-by: Pallavi G <pallavi.g@intel.com>
Signed-off-by: Yogesh M <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v2 changes)
[danvet: Reorder if-ladder to avoid two IS_VLV checks.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>