drm/i915/dp: use the sink rates array for max sink rates

Looking at DPCD DP_MAX_LINK_RATE may be completely bogus for eDP 1.4
which is allowed to use link rate select method and have 0 in max link
rate. With this change, it makes sense to store the max rate as the
actual rate rather than as a bw code.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/3e8baadb406d59f414cab36fed9f0b35d207fde5.1491485983.git.jani.nikula@intel.com