drm/dp: Add DPCD defines for register 0x007

Low four bits are downstream port count.  High bit indicates peer OUI
support.  OUI matching will allow us to do additional per-sink handling
for things like DP->VGA bandwidth limits or (hopefully) the iMac-as-
display hack.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>