drm/i915: Enable 5.4Ghz (HBR2) link rate for Displayport 1.2-capable devices

For HSW+ platforms, enable the 5.4Ghz (HBR2) link rate for devices that support it. The
sink device must report that is supports Displayport 1.2 and the HBR2 bit rate in the
DPCD in order to use HBR2.

Signed-off-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>