drm/i915/cnl: Fix DP max voltage

On clock recovery this function is called to find out
the max voltage swing level that we could go.

However gen 9 functions use the old buffer translation tables
to figure that out. That table is not valid for CNL
causing an invalid number of entries and an invalid selection
on the max voltage swing level.

v2: Let's use same approach that previous platforms.
v3: Actually use n_entries and avoid duplicated -1.
v4: Avoid cnl_max_level and use current style.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170831145356.15932-1-rodrigo.vivi@intel.com