drm/i915/skl: Add support for edp 1.4 intermediate frequencies

eDp 1.4 supports custom frequencies.
Skylake supports following intermediate frequencies : 3.24 GHz, 2.16 GHz and
4.32 GHz along with usual LBR, HBR and HBR2 frequencies.
Read sink supported frequencies and get common frequencies from sink and
source and use these for link training.

v2: Rebased, removed calculation of min_clock since for edp it is taken as
max_clock (as per comment).
v3: Keeping single array for link rates (Satheesh)
v4: Setting LINK_BW_SET to 0 when setting LINK_RATE_SET (Satheesh)
v5: Some minor nits (Ville)
v6: Keeping separate arrays for source and sink rates (Ville)
v7: Remove redundant setting of DP_LINK_BW_SET to 0 (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>