drm/i915/bxt: fix DDI PHY vswing scale value setting

According to bspec the DDI PHY vswing scale value is "don't care" in
case the scale enable bit [27] is clear. But this doesn't seem to be
correct. The scale value seems to also matter if the scale mode bit
[26] is set. So both bit 26 and 27 depend on the value. Setting the
scale value to 0 while either bit is set results in a failed modeset on
HDMI (sink reports no signal).

After reset the scale value is 0x98, but according to the spec we have
to program it to 0x9a. So for consistency program it always to 0x9a
regardless of the scale enable bit.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Tested-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>