drm/dp: DPCD register defines for link status within ESI field

Link status is available in the ESI field on devices with DPCD r1.2 or
higher. DP spec also says "An MST upstream device shall use this field
instead of the Link/Sink Device Status field registers, starting from DPCD
Address 00200h."

v2: Prefixed DP_ (Jani)
    Rewrote commment to stay within 80 cols.
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914062127.12047-1-dhinakaran.pandiyan@intel.com