drm/i915/skl: Read sink supported rates from edp panel

v2: Using DP_SUPPORTED_LINK_RATES macro for supported_rates array (Satheesh).
v3: Reading dpcd's supported link rates tables based upon edp version in the
same patch.
v4: Move version check under is_edp (Satheesh)
v5: Using le16 for rates, some naming, and removing nested if block (Ville)
v6: Correctly using DP_MAX_SUPPORTED_RATES and removing DP_SUPPORTED_LINK_RATES
(Ville)
v7: Incorrectly removed DP_SUPPORTED_LINK_RATES in v6, re-adding it
v8: Checking return value of intel_dp_dpcd_read_wake() (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>