drm/i915/dp: update training set in a burst write with training pattern set

The DP spec allows this, and requires it when full link training is
started with non-minimum voltage swing and/or non-zero pre-emphasis.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>