drm/i915: Pass lane count to bxt_ddi_phy_calc_lane_optmin_mask()

Pass lane count to bxt_ddi_phy_calc_lane_optmin_mask() instead of having
it extract that number from a pipe_config to decouple the phy code from
intel_crtc_state.

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/a4977e0207e594953c4f9d1b5f2ef972a8679e74.1475770848.git-series.ander.conselvan.de.oliveira@intel.com