drm/i915/dp: Add HBR3 rate (8.1 Gbps) to dp_rates array

dp_rates[] array is a superset of all the link rates supported
by sink devices. DP 1.3 specification adds HBR3 (8.1Gbps) link rate
to the set of link rates supported by sink. This patch adds this rate
to dp_rates[] array that gets used to populate the sink_rates[]
array limited by max rate obtained from DP_MAX_LINK_RATE DPCD register.

v2:
* Rebased on top of Jani's localized rates patch

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1519857110-26916-1-git-send-email-manasi.d.navare@intel.com