drm/i915: Introduce intel_ddi_dp_voltage_max()

Rather than sprinkling ideas of how big the DDI buf translation tables
are somewhere in intel_dp.c, let's concentrate it all in intel_ddi.c
where the actual tables are defined. To that end we introduce
intel_ddi_dp_voltage_max() which will actually look at the proper
translation table to determine what is the maximum voltage swing level
supported.

v2: Mask out the preemphasis bits from the return value of
    intel_ddi_dp_voltage_max()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223174901.26749-1-ville.syrjala@linux.intel.com
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 9b5fcc1..c3c8ed7 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -1233,6 +1233,8 @@ void intel_ddi_clock_get(struct intel_encoder *encoder,
 			 struct intel_crtc_state *pipe_config);
 void intel_ddi_set_vc_payload_alloc(struct drm_crtc *crtc, bool state);
 uint32_t ddi_signal_levels(struct intel_dp *intel_dp);
+u8 intel_ddi_dp_voltage_max(struct intel_encoder *encoder);
+
 unsigned int intel_fb_align_height(struct drm_i915_private *dev_priv,
 				   unsigned int height,
 				   uint32_t pixel_format,