drm/i915: Clear leftover DP vswing/preemphasis values before modeset

Currently we clear the leftover vswing/preemphasis values only
at the start of link training. That means the initial vswing
programming performed during modeset is going to use stale values
left over from the previous link training sequence, and then at
the start of link training we're going to reset the levels back
to 0. Seems much better to make sure we start with level 0 from
the get go.

Additionally if LTTPRs are present the leftover vswing/preemphasis
values are those of the last link in the chain, so not the values
that our PHY is even using after a successful link training sequence.

So let's make sure everything is cleared up before we start
programming anything.

Suggested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930134310.31669-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>