drm/dp: Add HBR3 support in existing DRM DP helpers

Existing helpers add support upto HBR2. This patch
adds support for HBR3 rate (8.1 Gbps) introduced as
part of DP 1.4 specification.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1516660991-20697-1-git-send-email-manasi.d.navare@intel.com