drm: Add DPCD definitions for DP 1.4 DSC feature

Display stream compression is supported on DP 1.4 DP
devices. This patch adds the corersponding DPCD
register definitions for DSC.

v4:
* Add DSC Enable DPCD register def (Ander)
v3:
* Add some SHIFTS and MASKS for uniformity (Jani Nikula)
v2:
* Rebased on drm-tip

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1491259870-25613-1-git-send-email-manasi.d.navare@intel.com