drm/i915/dp: fix up VLV DP handling v2

Needed to handle pre/post enable/disable paths on VLV and avoid a few
fields that are marked reserved on VLV.

v2: don't set color range or DP PLL fields (Jani)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>