drm/dp: Update DPCD defines

Sources: DP, eDP, and DP interop specs, and a VESA slideshow about DP
1.2 for the MST bits.

Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>