drm/dp: Document DP spec versions for various DPCD registers

Note with a comment anything newer than DP 1.1a.

Obviously this needs some work still...

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>