drm/i915: decouple gen9 and gen10 dp signal levels.

Let's decouple bxt, glk and cnl dp signal levels
from other DDIs to avoid confusion.

No functional change. Only a reorg to avoid messing
with currently working DP signal levels when
moving voltage swing sequences around to match spec.

v2: ddi_signal_levels is also called from other ddi
    platforms, so don't remove IS_GEN9_BC check from
    skl_ddi_set_iboos. (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.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/20170829232230.23051-2-rodrigo.vivi@intel.com