drm/i915: Move broxton vswing sequence to intel_dpio_phy.c

The vswing sequence is related to the DPIO phy, so move it closer to the
rest of DPIO phy related code.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/59aa5c85a115c5cbed81e793f20cd7b9f8de694b.1475770848.git-series.ander.conselvan.de.oliveira@intel.com