drm/dp: Add definition for Display Control DPCD Registers capability size

This is used when reading Display Control capability Registers on the sink
device.

cc: dri-devel@lists.freedesktop.org
Signed-off-by: Yetunde Adebisi <yetundex.adebisi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459865452-9138-2-git-send-email-yetundex.adebisi@intel.com