drm/i915/bxt: Set DDI PHY lane latency optimization during modeset

So far we configured a static lane latency optimization during driver
loading/resuming. The specification changed at one point and now this
configuration depends on the lane count, so move the configuration
to modeset time accordingly.

It's not clear when this lane configuration takes effect. The
specification only requires that the programming is done before enabling
the port. On CHV OTOH the lanes start to power up already right after
enabling the PLL. To be safe preserve the current order and set things
up already before enabling the PLL.

v2: (Ander)
- Simplify the optimization mask calculation.
- Use the correct pipe_config always during the calculation instead
  of the bogus intel_crtc->config.

CC: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95476


Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>