drm/i915/cnl: Implement voltage swing sequence.

This is an important part of the DDI initalization as well as
for changing the voltage during DisplayPort link training.

This new sequence for Cannonlake is more like Broxton style
but still with different registers, different table and
different steps.

v2: Do not write to DW4_GRP to avoid overwrite individual loadgen.
    Fix PORT_CL_DW5 SUS Clock Config set.
v3: As previous platforms use only eDP table if low voltage was
    requested.
v4: fix Werror:maybe uninitialized (Paulo)
v5: Rebase on top of dw2_swing_sel changes
    on previous patches.
v6: Using flexible SCALING_MODE_SEL(x).

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497047175-27250-11-git-send-email-rodrigo.vivi@intel.com