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

include/drm/drm_dp_helper.h

@@ -334,6 +334,7 @@
 # define DP_LINK_BW_1_62		    0x06
 # define DP_LINK_BW_2_7			    0x0a
 # define DP_LINK_BW_5_4			    0x14    /* 1.2 */
+# define DP_LINK_BW_8_1			    0x1e    /* 1.4 */
 
 #define DP_LANE_COUNT_SET	            0x101
 # define DP_LANE_COUNT_MASK		    0x0f