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