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
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index d3e5fdf..49a8c33 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -3506,13 +3506,11 @@ intel_dp_set_signal_levels(struct intel_dp *intel_dp)
 	uint32_t signal_levels, mask = 0;
 	uint8_t train_set = intel_dp->train_set[0];
 
-	if (HAS_DDI(dev_priv)) {
+	if (IS_GEN9_LP(dev_priv) || IS_CANNONLAKE(dev_priv)) {
+		signal_levels = bxt_signal_levels(intel_dp);
+	} else if (HAS_DDI(dev_priv)) {
 		signal_levels = ddi_signal_levels(intel_dp);
-
-		if (IS_GEN9_LP(dev_priv) || IS_CANNONLAKE(dev_priv))
-			signal_levels = 0;
-		else
-			mask = DDI_BUF_EMP_MASK;
+		mask = DDI_BUF_EMP_MASK;
 	} else if (IS_CHERRYVIEW(dev_priv)) {
 		signal_levels = chv_signal_levels(intel_dp);
 	} else if (IS_VALLEYVIEW(dev_priv)) {