drm/i915/skl: Add support for edp1.4 low vswing

Based upon vbt's vswing preemph settings value select the appropriate
translations for edp.

v2: Incorporating bspec changes for vswing and preemph levels, adding edp
translation table. Removed HSW from selection 9 which is specific to skl and
correcting the returning of level2 from max pre emph (Damien)

v3: Rebasing on top of renaming patches. Adding level(3,0) since level(2,2) as
mentioned in bspec is invalid as per edp spec. Also changed the determining of
size of the table selected (Satheesh).

v4: Adding level 3 in max voltage selection if low vswing is selected (Satheesh)

v5: Add a comment stating that skl_ddi_translations_edp is for eDP 1.4
    low vswing panels.

v6: Updating recommended DDI translation table for edp 1.4

Reviewed-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v4)
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com> (v6)
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>